Substrate inhibitions that manifest within the cometabolism system of 4-chlorophenol (4-cp) and phenol were alleviated through the application of granular activated carbon (GAC) in batch biodegradation. It was found that 4-cp was preferentially adsorbed over phenol by the GAC and that 50% to 70% of the adsorption was achieved within the first two hours of contact. The kinetics of 4-cp adsorption was also much faster than that of phenol, even when the co-existing phenol was of a significantly higher initial concentration. As a result, competitive inhibition between the two compounds was minimized. Adsorption also caused a lowering of the phenol concentration in solution with a concomitant reduction in the substrate inhibition effect on cell growth. The addition of GAC benefited the biotransformation process through shortening the total degradation time for 600 mg L-1 phenol and 100 mg L-1 4-cp from 42 h to 12 h; and it also made it possible for cells to survive and transform 600 mg L-1 phenol and as high as 400 mg L-1 4-cp in free suspension cultures. Repeated operations in which GAC was reused showed that GAC could be regenerated by the cells, thus rendering the GAC incorporated process amenable to long term operations.