Biotransformation of 2-chlorophenol (2-cp) and 4-chlorophenol (4-cp) in the presence of phenol by Pseudornonas putida (ATCC 49451) was investigated. Strain ATCC 49451 was unable to utilize 2-cp and 4-cp as the sole carbon and energy source. In the presence of phenol as a growth substrate, 2-cp and 4-cp could be transformed through cometabolism. It was found, however, that cell growth and phenol degradation were strongly inhibited by the presence of 2-cp and 4-cp. A much longer lag phase (19 h versus 3 h) occurred with the mere addition of 40 mg/L 2-cp and 100 mg/L 4-cp. Further increase in 2-cp and 4-cp concentrations resulted in incomplete transformation: only 80% of the initial 100 mg/L 4cp and 50% of the initial 40 mg/L 2-cp could be degraded in the presence of 200 mg/L phenol. Interactions between substrates affected cell growth and substrates degradation significantly and both 2-cp and 4-cp were toxic to the cells. Kinetic models for cell growth as well as substrate transformation were established to simulate the experimental data. The form of the kinetic models and magnitude of the model parameters (K-2 = 5.62 mg/L > K-3 = 3.57 mg/L; k(d2) = 17.8 mg/L < k(d3) = 51.5 mg/L) indicate that 2-cp and 4-cp exhibited different inhibition and toxicity effects on the cells and their degradation capacities. Kinetics also revealed that the toxicity effect of the chlorophenols dominated over the competitive inhibition effect.