LATE goals from Graham Dorrans and Ishmael Miller saw professional Albion tighten their grip on second place at promotion-rivals Swansea.
For the second time in three days, Dorrans set up the victory when he followed up his spot-kick clincher against Blackpool by breaking the deadlock with another superb 12-yard conversion 11 minutes from time - his 15th of the season.
Sub Giles Barnes again proved the architect as, just like on Saturday, it was his trickery which earned the penalty.
And sub Miller ensured City were heading for only their third home defeat of the season when he raced clear in the 89th minute and slotted home his second goal in as many games.
This mature performance by Roberto Di Matteo's men earned Albion one of their biggest results of the season.
They restricted the hosts to few opportunities and, when their chances came, snatched them with both hands.
Di Matteo made three changes, with Luke Moore, Ben Watson and Abdoulaye Meite coming in for benched duo Miller and James Morrison plus Steven Reid (groin).
Swansea dropped Baggies old boy Craig Beattie, Ashley Richards, Tom Butler and David Cotterill, with Shefki Kuqi, skipper Gary Monk, Nathan Dyer and Cedric van der Gun earning recalls.
Albion started the day in the second automatic promotion berth, some two places and nine points better off than the hosts.
Both teams made a cagey start and the first effort on goal did not arrive until the sixth minute, when Monk was well off target with a header from Mark Gower's free-kick.
With both teams packing the midfield, there was plenty of activity in the middle third of the field but little in the way of goalmouth chances.
However, Youssouf Mulumbu was guilty of presenting Swansea with a half-chance on a plate in the 13th minute when he was dispossessed by Dyer on the edge of his own box.
Thankfully for the visitors, the pint-sized winger curled a right-footed effort harmlessly over Scott Carson's bar.
After successfully negotiating the opening 20 minutes without any real alarms, Albion began to grow into the game, although several attacks lost their impetus when confronted by the division's meanest defence.
Albion had a minor scare in the 22nd minute when Dyer's miscued cross struck the outside of the near post, although Carson had it covered.
For all of the Baggies' endeavour, they had not recorded a single shot on goal as the game passed the half-hour mark.
But Swansea weren't exactly peppering Carson's goal either and their next effort - a 20-yarder from van der Gun on 31 - went well wide.
Albion's most dangerous moment thus far came in the 38th minute.
Robert Koren, Chris Brunt and Dorrans combined well through the middle of the park before the latter fizzed a dangerous ball across the face of goal - but no-one was there to tap it in.
Moments later, there was an almost identical incident at the other end after Meite was outmuscled by Kuqi.
Gower whipped in a terrific left-wing cross which just evaded Dyer's outstretched boot at the near post.
This chess-like half came to an end when Carson easily collected Angel Rangel's weak 25-yard grass-cutter.
Di Matteo rang an enforced change at half-time as Gianni Zuiverloon came on for the injured Jonas Olsson. The former Dutch Under-21 international slotted in at right-back, with Gabriel Tamas moving across to centre-half.
Albion started the second 45 brightly and almost snatched a 49th-minute lead with their first effort on goal.
A terrific spell of keep-ball culminated in Dorrans charging at the Swans defence and finding Moore, who drew a terrific near-post save from Dorus de Vries with an acute 15-yard drive.
It was now the Baggies who were dictating the tempo of the game, with the hosts struggling to get near the ball.
However, chances remained at a premium in South Wales.
And Carson had little trouble dealing with a long-range Alan Tate effort which packed power but flew straight at the England international.
On 63, Dyer jinked past a couple of Baggies defenders before curling the ball straight at Carson from 18 yards.
However, had the ex-Southampton player found the net, referee Roger East would have had to contend with angry Albion appeals for a foul on Mulumbu in the build-up.
Di Matteo made his second change on 65, replacing Moore with Miller.
And he made his final switch five minutes later, bringing on Giles Barnes for Brunt.
Miller was soon in the thick of it and it took a last-ditch challenge by Tate to prevent him troubling de Vries' goal.
From Dorrans' ensuing corner, Tamas was well off target with a presentable headed chance.
Miller's fellow sub Barnes was at the centre of the game's biggest moment on 79 when he won a penalty after being tripped by Rangel in the box.
Ice-cool Dorrans made no mistake, clinically curling his spot-kick past de Vries, who had guessed the right way.
And Miller put the result beyond doubt in the 89th minute when he raced onto Dorrans' through-pass, jinked past de Vries - twice - before smashing the ball past the defender on the line.
SWANSEA (4-5-1): De Vries; Rangel, Monk, Williams, Tate; Dyer (Cotterill 80), Gower (Beattie 80), Britton (Bauza 86), Pratley, van der Gun; Kuqi. Subs not used: Cornell (gk), Richards, Pintado, Trundle.
ALBION (4-5-1): Carson; Tamas, Meite, Olsson (Zuiverloon HT), Cech; Koren, Watson, Mulumbu, Dorrans, Brunt (Barnes 70); Moore (Miller 65). Subs not used: Kiely (gk), Morrison, Cox, Mattock.
GOALS: ALBION - Dorrans (pen 79), Miller (89).
BOOKINGS: SWANSEA - Monk (foul 90). ALBION - Dorrans (foul 30), Miller (unsporting behaviour 90).
REFEREE: R East (Wiltshire).
ALBION STARMAN: Marek Cech.