Orange County’s office vacancy rate dipped below the psychologically key threshold of 10 percent in the fourth quarter, Voit Commercial Brokerage LP said Tuesday.

The vacancy rate was 9.6 percent at the end of 2004, its lowest level since 2000, according to Voit. It was 10.5 percent in the third quarter and 12.4 percent at the end of 2003.

O.C.’s average vacancy has been decreasing for the past several quarters. Breaking the 10 percent mark is significant because it usually means space is limited enough to force office rents up, according to industry sources.

Vacancy is declining as a result of more companies buying and leasing space than vacating it, which brokers call “absorption.” Absorption totaled 689,260 square feet in the fourth quarter and 3 million for all of 2004, the 10th consecutive quarterly gain.

Industrial absorption was even better. It totaled 1.8 million square feet in the fourth quarter and 4.3 million square feet for 2004.

The industrial vacancy rate dropped to 4.3 percent, down from 6.1 percent a year ago.