SACRAMENTO — Corporations have contributed a steadily decreasing portion of the state's property taxes since Proposition 13 was approved three decades ago, even as the share paid by homeowners has risen, according to a report released Thursday by a tax-reform group.

The disparity has potentially cost the state millions of dollars, the report by the California Tax Reform Association concluded.

Examining newly obtained county numbers often stretching back decades, the report showed a shift in the tax burden in all but a handful of counties. In Santa Clara County, the share paid by residents went from 50 percent to 64 percent.

Proposition 13 capped how much property taxes can increase each year unless a property changes hands. The report suggests that corporations have taken advantage of the state's Byzantine property-transfer rules to keep from having land reassessed more frequently.

The report's release comes days before the first legislative hearing on a bill by Assemblyman Tom Ammiano, D-San Francisco, that seeks to tighten those rules.

Unlike when homes are bought and sold, it can be less clear when commercial and industrial property changes hands, putting the burden on county assessors to track stock sales and news accounts of mergers. Another complication is when several firms divvy up stakes in a company, confounding state law that says property changes hands only after one entity obtains majority control.

"The

law just doesn't fit," said Lenny Goldberg, the report's author.

He said assessors, for example, have been slow to reassess bank branches following the mergers of Chase and Washington Mutual and of Wachovia and Wells Fargo.

Santa Clara County Assessor Larry Stone bristled at the idea that assessors are leaving hundreds of millions of dollars on the table. He noted that reassessments were done locally after Chevron bought Texaco and when Hewlett-Packard took over Compaq.

"When we have a major acquisition or merger, we reassess — and we've been doing that for years," said Stone, who hadn't gone through Goldberg's report.

Jon Coupal, president of the Howard Jarvis Taxpayers Association, said sticking California businesses with higher property taxes would be a mistake.

"That eventually filters down to higher costs and fewer services and less employment," he said.

Goldberg countered that as strapped cities reduce services, there's too much money at stake to ignore: "We're just scratching the surface."

Contact Denis C. Theriault at 916-441-4651.