Voters approved Proposition 8 last year, which made gay marriages illegal. The court decides Tuesday if the measure was unconstitutional.
The decision comes as more states consider the issue, giving it a significance beyond California. Legislatures or courts in five other states – most recently in Iowa and Maine – have legalized same-sex marriage. New Hampshire and New York are weighing similar laws.
The California court will determine if Prop. 8, approved by 52 percent of voters last year, is an illegal revision of the state constitution. In addition, the justices are likely to determine the validity of some 18,000 same-sex marriages performed before last November's poll. The decision is expected to be posted on the court's website at 10 a.m. Pacific Time.
Gay marriage was legalized in California last year when the state's Supreme Court ruled, in a 4-to-3 decision, that a state law limiting marriage to a man and woman violated constitutional rights of gays and lesbians. While the ruling led to a wave of same-sex marriages in the state, it also gave rise to the $83 million campaign to amend the constitution that resulted in Prop. 8.