The 'Twilight' actor wants to speak with Liberty Ross to test Kristen Stewart's claims that she and film director Rupert Sanders had just a "momentary indiscretion."
A source told the Daily Mirror newspaper: "He needs to know for his own peace of mind if theirs was just a fleeting moment.
"He hopes Liberty can help him find the answers he so desperately craves."
Kristen, 22, said at the time: "This momentary indiscretion has jeopardized the most important thing in my life, the person I love and respect the most, Rob. I love him, I love him, I'm so sorry."
However 26-year-old Robert is said to be unconvinced and wants to hear from the mother of 41-year-old Rupert's two children.
The source added: "Nothing is very clear at the moment but perhaps a meeting would lend some clarity to the situation. He needs to know the truth."
Meanwhile, it has emerged that Kristen dumped the 'Snow White and the Huntsman' boss by text message hours after their liaison was caught on camera by US Weekly magazine.
A friend, who asked not to be named, said: "After that day she texted Rupert that she never wanted to do that again because she feared being caught and that things were too intense."
She had no intention of leaving Robert and Rupert has never mentioned walking away from his family.
"She knew she did wrong, but realized how those photos would rip out Rob's heart. She has been in floods of tears since."