Would Liam Hemsworth really cheat on Miley Cyrus?

A new report from Star magazine claims Hemsworth cheated on his superstar fiancee with "Mad Men" star January Jones at a pre-Oscar party this past weekend.

RadarOnline reports that the 23-year-old "Hunger Games" star showed up to the Chateau Marmont hotel sans Cyrus and cozied up to the 35-year-old single mother , who is recently single after splitting with boyfriend Noah Miller in January.

An eyewitness told the magazine the pair looked like they had some "hot chemistry," and "were all over each other -- and they even kissed."

The source claims the pair made an effort not to attract attention as they left together, staggering their exits, but it didn't work since photographers snapped photos of both actors together in a car .

Hemsworth and Cyrus became engaged in June, but sources told the magazine that when the 20-year-old pop star hit up Elton John's annual Oscar party on Sunday all by her lonesome , she was "all over the place" and "gave off the impression that shes single." And when someone asked how Hemsworth was doing the insider claims Cyrus said she didn't care how he was doing.

It's not the first time Hemsworth has been accused of cheating on his fiancee . Last fall, sources claimed they spotted the Australian actor getting "very cozy" with a hot blonde at the Samsung Galaxy Note II party in Los Angeles on Oct. 25.

Requests for comment made by The Huffington Post to Hemsworth's rep have not been returned at this time.