TEN-MAN Albion's valiant efforts proved in vain as two second-half goals ended their ten-game unbeaten start to the season.
The battling Baggies were more than a match for the Gunners' talented young guns but Sanchez Watt's 68th-minute strike and substitute Carlos Vela's tap-in seven minutes later saw them exit the Carling Cup at the third-round stage.
The visitors started well in front of more than 3,500 travelling fans - and the prospect of a memorable victory at the Emirates Stadium became more and more plausible.
After Arsenal's Gilles Sanu hit the post, Simon Cox saw a header cleared off the goalline in the 20th minute as Roberto Di Matteo's men got on top.
But Albion's job was made all the harder seven minutes before half-time when ex-Gunner Jerome Thomas was sent-off for violent conduct.
Even with a one-man disadvantage, the Baggies looked comfortable and Cox missed a golden opportunity to fire them ahead before two goals out of the blue booked the Barclays Premier League outfit's place in the fourth-round draw.
Gonzalo Jara was handed his Albion debut at left-back in the wake of ankle injuries to Joe Mattock and Marek Cech.
Di Matteo made six changes in total as Robert Koren, Filipe Teixeira, Dean Kiely, Leon Barnett and Cox were recalled to the starting XI.
Arsene Wenger named a young Gunners side but opted for an experienced centre-half duo in Philippe Senderos and Mikael Silvestre.
The hosts threatened first in the third minute as Senderos was allowed to run free at the near post from Jack Wilshere's left-wing corner.
Fortunately for Albion, the Swiss international headed well over.
But the visitors were guilty of the same mistake just two minutes later when, from another Wilshere flag-kick, Senderos got in front of Barnett, only to again head wide of the right-hand post.
The Baggies soon settled into their stride and their first effort on goal came in the 16th minute.
Teixeira chested down Graham Dorrans' inch-perfect diagonal ball for Koren whose skidding 18-yard strike flew a yard past the left-hand post.
Two minutes later, Kieran Gibbs centred from the left and Watt exchanged passes with Wilshere, only to see his shot hit the base of the right-hand post before Kiely smothered the ball.
Within seconds, the Baggies turned defence into attack and Luke Moore squared for Cox who turned on the edge of the area before firing a low shot through Silvestre's legs, forcing Wojciech Szczesny into a save to his right.
Cox turned supplier in the 19th minute when he found Koren just outside the box and the Slovenia skipper swivelled before forcing a fine stop from Szczesny low to his right.
From the resulting Dorrans corner, Cox's header was cleared off the line by Kerrea Gilbert and Olsson nodded the rebound wide of the left-hand post.
Wilshere tried his luck with a 25-yard effort in the 31st minute but saw the ball fly well wide of the right-hand upright.
Two minutes later, Szczesny dropped Dorrans' free-kick under pressure from Barnett and Gibbs' attempted clearing header rebounded off Moore and just wide of the empty goal.
Thomas left Sanu and Gilbert for dead in the 37th minute to race to the by-line before firing a ball across the six-yard box, which Szczesny managed to grasp before Moore could get a touch.
A minute later, Thomas' night was turned on its head when, following a challenge with Wilshere, he pushed the Gunners midfielder in the face and was sent-off by referee Lee Mason.
Five minutes before the break, Francis Coquelin threaded Watt in on goal and Kiely managed to block his effort before Zuiverloon cleared the danger.
Albion's ten-men managed to reach half-time goalless, giving Di Matteo the chance to reorganise his side.
The visitors came back out with the same line-up, with Moore still filling in at right midfield and Cox playing as a lone frontman.
It was difficult to tell which team had ten men as Albion started the second half in confident mood - and they created a glorious opportunity in the 58th minute.
The energetic Dorrans pulled the ball back for Zuiverloon and his exquisite cross was headed agonisingly over by Cox from eight yards.
Moments later, Wenger made a double change as Mark Randall and Vela came on for Francis Coquelin and Sanu respectively.
But still Albion pressed and, on the hour mark, Dorrans found Cox on the left and the ex-Swindon marksman cut inside before drilling a low 20-yard shot straight at Szczesny.
A minute later, Cox made way for Youssouf Mulumbu.
The ten-man Baggies were putting up stern resistance - but that came undone courtesy of a 68th-minute bolt out of the blue.
Ramsay found Vela inside and the Mexican wriggled free of Olsson before forcing Kiely into a one-handed save to his left.
But Watt was quickest to the rebound and had the simple task of slotting into an empty net.
Kiely was in the action again in the 72nd minute when he snaffled Ramsey's deflected 20-yard strike.
Moore found himself one-on-one with Senderos in the 74th minute and he managed to manufacture enough space to force Szczesny into a save at the foot of his right-hand post.
That was the ex-Villa striker's final contribution as he was replaced by Chris Wood.
Albion were still very much in the tie but shot themselves in the foot in the 75th minute.
Substitute Randall's delicious 20-yard chip beat Kiely and hit the crossbar.
Barnett had the chance to clear the danger but tried to chest the ball back to Kiely, allowing Vela to nip in and bundle the ball home.
Teixeira picked out Wood with a ball over the top ten minutes from time but the New Zealand international's first-time effort on the half-volley flew just wide.
The Gunners then took control of the game and ran the clock down with a spell of keep ball, leaving Albion to reflect on a night of what might have been.
ARSENAL (4-4-2): Szczesny; Gilbert, Senderos, Silvestre, Gibbs; Wilshere, Coquelin (Randall 58), Ramsey, Traore (Barazite 78); Watt, Sanu (Vela 58). Subs not used: Shea (gk), Bartley, Eastmond, Frimpong.
ALBION (4-4-2): Kiely; Zuiverloon, Barnett, Olsson (Meite 80), Jara; Thomas, Dorrans, Koren, Teixeira; Cox (Mulumbu 61), Moore (Wood 74). Subs not used: Carson (gk), Martis, Bednar, Reid.
GOALS: ARSENAL - Watt (68), Vela (75).
BOOKINGS: ARSENAL - Senderos (foul 32), Ramsey (foul 57). ALBION - Cox (dissent 38).
SENDING-OFF: ALBION - Thomas (violent conduct 38).
REFEREE: L Mason (Lancashire).
ALBION STARMAN: Graham Dorrans.