The former 'That '70s Show' stars have reportedly been dating but are also seeing other people.
The 'Ted' actress and the 'Two and a Half Men' actor were spotted riding around Hollywood on Ashton's motorcycle on June 17.
The screen stars were also spotted sharing meals several times during the spring and apparently took a three-day road trip getaway outside Los Angeles in late April - but are not an official couple.
A source told UsMagazine.com: ''They're not exclusive, but they are hanging out and seeing where it's going.''
For six years, 34-year-old Ashton was married to 49-year-old Demi Moore.
At the same time 28-year-old Mila was in a long-term relationship with 31-year-old actor Macaulay Culkin.
A second insider added: ''They've been hot for each other for years. It stretches back to when they were friends on their show!''
Mila is still in touch with the marine she went on a date with last year.
The 'Black Swan' actress accepted an invitation sent by Sergeant Scott Moore of the Third Batallion, 2nd Marines to his Marine Corps Ball via YouTube in late 2011, and she admits they continue to speak on the phone occasionally.
She said: ''We are still in contact. We email back and forth and spoke on the phone about a month ago!''