Two goals in six second-half minutes sent West Brom back into the Coca-Cola Championship top two and left Peterborough in relegation strife.
Mark Cooper's men frustrated Albion for an hour at the Hawthorns but a Luke Moore strike and an own goal from Posh centre-half Ryan Bennett ensured that Roberto Di Matteo's side moved back into second place.
Newcastle, Nottingham Forest and Cardiff all failed to win in early kick-offs on Boxing Day and the double salvo proved enough to leave Albion eight points behind the Magpies.
The hosts started well and Jerome Thomas had the first sight of goal with a powerful volley from the edge of the area but Aaron Mclean was well-placed to block his stinging drive.
Albion were controlling possession but struggled to find a killer pass in the final third as a disciplined Posh side made life difficult for Di Matteo's side.
Moore finally called goalkeeper Joe Lewis into action on the half-hour mark after the ball deflected off Peterborough midfielder Lee Frecklington to inadvertently find him in space but the England Under-21 stopper stood up well and saved with his feet.
Albion were firmly on top and should have gone ahead before the break when right-back Gianni Zuiverloon slashed wildly over as a corner was not dealt with.
Chris Whelpdale fired wide at the other end at the culmination of a rare break for the visitors as Albion found themselves booed off at the break.
Thomas looked the most likely architect of an opener and twice he saw excellent crosses evade Simon Cox before Abdoulaye Meite wasted a great chance.
Chris Brunt sent in a corner from the right but despite rising unmarked six yards from goal Meite could not direct his header on target.
It was a let-off for the visitors but Moore had the hosts ahead moments later in the 61st minute.
Brunt started the move when he played the ball to Graham Dorrans on the halfway line and the Scot carried the ball forward past several challenges.
He then played Moore in with a clever reverse pass and the former Aston Villa man took a touch before sending in a left-footed shot which took a deflection on its way past the dive of Lewis.
Albion were all over the strugglers now and saw Bennett hand them a second in the 67th minute.
Dorrans fired a shot into the turf and Bennett - under pressure from Cox - stretched to send a header past Lewis and into his own net.
Shaun Batt's 25-yard shot flew wide as Posh staged a late rally but Albion were comfortable as they held on for all three points.
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