The first round of voting, which ends on 12 January, will decide which films and actors will be nominated.
Another round of voting will then take place to determine who will pick up the prestigious golden statuettes at the ceremony in Los Angeles on 24 February.
The award field is thought to be wide open, with Atonement and Into the Wild among the possible contenders.
Joel and Ethan Coen's No Country for Old Men is also one of the early favourites - but there is no clear frontrunner.
There is also uncertainty over what form the ceremony will take if the current Hollywood writers' strike carries on.
There could be no script for host Jon Stewart and the star presenters, and the strike could also affect the event's big-name turnout.
If writers picket the event, few A-list actors would want to be seen crossing the picket line.
Members of the Writers Guild of America have been on strike for seven weeks, but there are another two months until the Academy Awards take place at the Kodak Theatre in Hollywood.
The nominees and winners are decided by members of the Academy of Motion Picture Arts and Sciences, which includes actors, directors and the cream of professionals from across the movie industry.
The ballots were sent on 26 December by auditing firm PricewaterhouseCoopers, who will also count the returned papers under strict security.
The initial votes must be returned by 1700 Los Angeles time on 12 January, with the nominations made public 10 days later.