A discontinuous tank reactor was used to eliminate 4-chlorophenol using soybean peroxidase (SBP) and hydrogen peroxide in an attempt to reduce the concentrations of buffer solutions used and to avoid adding polyethylene glycol. The stirred-tank reactor was operated in different operating conditions, and the influence of enzyme and substrate concentrations on the elimination of 4-chlorophenol was determined. Most of the assays provided good elimination percentages, which reached almost 90% for an emyme concentration of 7.5-10(-3) mg cm(-1) and a stoichiometric ratio of 1:1 for both substrates, when concentrations of 1.5 mmol dm(-3) or less were used. This work is a first step in the study of the SBP/4-chlorophenot system in. order to observe the influence of the reagents concentrations when no polyethylene glycol is added.