Shamir Fenelon netted his fourth goal of the pre-season campaign but it was not enough as Brighton & Hove Albion went down at League One Posh.
Despite the result, there was good news with the sight of Andrew Crofts - making his first start since January - leading the side out in the absence of Gordon Greer, while Chris O'Grady made his Seagulls debut.
There was further reason to be cheerful, just six minutes after kick-off, when Fenelon met O'Grady's pass and he skipped past two challenges before drilling home from close range.
O'Grady, who had made a bright opening, also went close with a powerful header from a Crofts cross.
Peterborough looked dangerous on the break and they twice went close in quick succession.
Their first effort arrived on 22 minutes when Jermaine Anderson's shot was deflected into the path of Kenny McEvoy, but he hit the upright.
Then Lewis Dunk made an excellent tackle to halt Britt Assombalonga, with Casper Ankergren saving from Conor Washington's follow-up.
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With temperatures touching 28c, both sides welcomed a second drinks break, and from the re-start Assombalonga should have done better with a header from McEvoy's cross.
Albion had the first attack of the second half with O'Grady flashing a header across the face goal from Bruno's right-wing cross.
However, it was Peterborough who got themselves back on level terms in the 56th minute when Assombalonga powered home a header from Jon Taylor's corner.
Posh made a raft of changes and they doubled their account on 64 minutes when Kane Ferdinand sent in a low cross from the right and Kyle Vassell touched the ball home.
Sub Kazenga LuaLua hit a free-kick straight at Joe Day, as Albion tried to work a way back in the game, and he also hit a trademark drive that the keeper was equal to.
Craig Mackail-Smith received a warm round of applause as he entered the fray against his former club, and his half-chance was the last opportunity for Albion as Posh held on for the win.