Katy Perry flirted with John Mayer at a barbecue bash on Monday.

The 28-year-old "Firework" singer - who split from the 35-year-old singer/songwriter for the second time in March - invited him to celebrate Memorial Day with her and several other friends at her new Hollywood Hills home, where they appeared to be back together. "They seem to be back together -- or back together-ish. They were very flirty and having a really good time together. They have good chemistry," a source said. Another eyewitness added, "They were definitely friendly."

Other guests at the party included Girls star Allison Williams and her boyfriend Ricky Van Veen, who were spotted on double dates with Katy and John in the past. Comedian Mindy Kaling, BJ Novak, 2 Broke Girls star Beth Behrs and Raising Hope actress Shannon Woodward also showed up to toast Katy's new home.

A source previously predicted that the on/off couple might reunite following their split in mid-March. "It's not over until it's over. You have to see how things play out," the insider added.