Biodegradation rates of 12 phenols were measured with respect to acclimated microbial biomass ranging from 23,000 to 23 x 10 to the 8th power cells/l. Rates ranged between 0.02 m/l/day for 1.6 mg/l p-bromophenol exposed to 23,000 x 10 to the fourth power cells/l and 1.41 mg/l/day for 3.2 mg/l p-methylphenol exposed to 2.3 x 10 to the 8th power cells/l. Generally, rates for all phenols were first-order in substrate concentration and zero-order in biomass concentration. Bromophenol biodegradation was preceded by lag periods of varying lengths and to a small extent the rate was dependent on microbial biomass. Results from the study suggest chemical biodegradation generally exhibits pseudo-first- and occasionally, second-order kinetics. (Copyright (c) 1989 Society for Industrial Microbiology.)