WEDNESDAY three times took the lead and could have scored more in a thriller but eventually were outgunned by League One’s top scorers after Darren Potter was sent off in the 69th minute.
For long periods the Owls’ prospects of earning an important victory for the club and Alan Irvine looked bright.
They went ahead 1-0, 2-1, then 3-2 with goals from Jermaine Johnson, Gary Madine and Chris Sedgwick.
But the red card was crucial. A mistake by Giles Coke saw possession lost in the Posh half: the home side counter-attacked and Potter was sent off for a foul committed when trying to halt the danger.
Wednesday came under pressure and saw their 3-2 lead turned into a 5-3 defeat - which meant six defeats in their last seven away games and increased speculation about Irvine’s future.
He made five changes - with no place in the 18 for Tommy Spurr or new loan signing Isaiah Osbourne.
Full backs Lewis Buxton and Mark Reynolds returned in place of Spurr and Daniel Jones.
Chris Sedgwick took over from Gary Teale on the wing, and Gary Madine and Neil Mellor replaced Clinton Morrison and Paul Heffernan up front.
Mellor had half a chance in the third-minute, shooting wide after a right-side corner from Darren Potter had brought headers from Reda Johnson and Michael Morrison.
But the breakthrough did come two minutes later: Giles Coke fed Jermaine Johnson and the winger typically ran at the defence to beat a couple of men and from a central position smashed home a drive to the keeper’s left.
Like the terrific goal against Hereford last Saturday it showed what a devastating player he is on his day.
Nicky Weaver denied Posh by touching a 30-yarder from George Boyd away from the top corner.
Johnson next took a diagonal pass from Potter, skinning his marker and pulling the ball back to Mellor, who was stretching as he knocked a difficult opportunity over.
Instead it was the home side who made the next mark. A powerful effort by Lee Tomlin forced Weaver to beat the ball out and Tomlin’s strike partner Craig Mackail-Smith followed up to score.
Another five minutes and another goal: a Coke effort from outside the box was deflected to Madine, who from 12 yards drilled a left-foot shot into the keeper’s bottom left-hand corner for his second goal for the Owls.
Wednesday were caught out in the 26th minute: Michael Morrison challenged an opponent but the ball ran to the unmarked Boyd; his well-struck shot from the edge of the box seemed to wrong-foot Weaver who stuck out his right leg and though his foot got a touch the shot flew into the net.
Wednesday went back in front in the 35th minute. Sedgwick intercepted a poor, square ball by Posh, found Mellor and reached a perfect return ball from the striker.
From the inside-right position, and the edge of the area, Sedgwick cracked a right-foot drive along the ground and in off the far post.
Peterborough would be glad to see the back of JJ after he failed to appear for the second half and was replaced by Teale.
Just when it seemed that the game was settling down and Wednesday were starting to tighten their grip, they went down to 10 men.
Coke tried to get the better of two opponents midway in the Posh half and lost possession. The Owls were hit on the break with Tomlin playing a through ball, Potter chasing back and lunging into a tackle from behind on Mackail-Smith. The ref consulted a linesman and then produced the red card, in the 69th minute.
Wednesday sought to stiffen their ranks by taking off Mellor and sending on James O’Connor - after he was booked for initially going on without permission.
But the home team again put the Owls under pressure in their box, they could not clear, and Boyd hit the roof of the net to equalise from 10 yards.
It took a superb clearance by Michael Morrison to deny Mackail-Smith.
But Wednesday were having to defend desperately and they were breached in the 84th minute.
Sub forward Nathaniel Mendez-Laing raced for a through ball: Reda Johnson went with him, Weaver was well positioned and the first shot was blocked.
But Mendez forced home the second ball despite despairing efforts by Weaver and Morrison.
In the dying minutes Weaver stopped a Tomlin shot with his feet.
With nothing to lose, the keeper went up for a corner in the fourth of five added minutes but Peterborough won possession and Mackail-Smith went clear to run the ball into the empty net.