The effect of vitamins on the aerobic degradation and dechlorination of 2-chlorophenol and 4-chlorophenol by Pseudomonas pickettii, strain LD1, and 4-chlorobiphenyl by Pseudomonas sp, strain CPE1 was determined. These microorganisms are capable of using the larger compounds as the sole carbon and energy source, bur do not need vitamins to metabolize them. The addition to the culture medium of a vitamin solution containing biotin, folic acid, pyridoxine hydrochloride, riboflavin thiamine hydrochloride, niacin, pantothenic acid, cyanocobalamin, p-aminobenzoic acid, and thioctic acid (total final concentration : less than or equal to 600 pbb) resulted in a 7%-16% increase in the amount of target compounds degraded over the incubation period required for the concentration of the compound in the cultures to drop to approximately zero. A corresponding increase in the amount of chloride ion produced was also detected during the same period, indicating active (and often stoichiometric) dechlorination of the target compounds.