Mixtures of palladium acetate and o-(di-tert-butylphosphino)biphenyl (4) catalyze the room-temperature Suzuki coupling of aryl bromides and aryl chlorides with 0.5-1.0 mol % Pd. Use of o-dicyclohexylphosphino)biphenyl (2) allows Suzuki couplings to be:carried out:at law catalyst loadings (0.000001-0.02 mol % Pd). The process tolerates a broad range of functional groups:and substrate combinations including the use of sterically hindered substrates. This is the most active Catalyst systeminterms of reaction temperature, turnover number, and steric tolerance which has been reported to date.