James Cameron and Kathryn Bigelow

Tonight's Oscars would normally be a shoo-in for James Cameron's Avatar, already the biggest-grossing movie in history. "Good box-office has historically been key to winning best picture, which usually goes to the movie with the first or second highest gross among the nominees," says one industry expert. The damper on Cameron's expectations is that this year the voting rules are different. With 10 instead of the traditional five nominees in the best picture category, voting will be a form of proportional representation instead of first-past-the-post winner-takes-all.

The academy's 5,800 voting members were asked to mark their ballots 1,2,3,4 and 5 in order of preference. Unless a picture has an outright majority, there will be recounts with the lowest placed pictures eliminated one by one and their second, third, fourth or fifth preferences redistributed until a winner emerges. This may favour the critically acclaimed The Hurt Locker, directed by Cameron's former wife Kathryn Bigelow.

People either like or loathe Avatar, which means it should top the first preferences but by nowhere near sufficient votes to win on the first count. Most people voting for Avatar are likely to give The Hurt Locker their second preferences whereas Avatar will probably get few second preferences. This would enable The Hurt Locker to reverse Avatar's triumph at the Golden Globes.

The only snag is that a row has erupted by attempts by one of The Hurt Locker's producers to canvas academy voters directly, which is prohibited by the rules. This may be enough to make Avatar favourite to triumph. Bigelow may have the consolation of winning best director.

Ordinary voting rules apply in the other categories, which should see Jeff Bridges (Crazy Heart) and Sandra Bullock (The Blind Side) repeat their Golden Globes acting triumphs, while Christoph Waltz (Inglourious Basterds) and Mo'nique (Precious) are virtually certainties in the best supporting acting categories. Tomm Moore's wonderful The Secret of Kells was a surprise nomination for best animated feature, but has no chance of beating Disney's Up.