The biological degradation potential on toluene of the bacteria Pseudomonas fluorescens isolated from soil, as influenced by humic acids, was investigated. The major environmental factors considered of interest for this experiment were temperature, pH of the media, and agitation (rpm), which were optimized using Response Surface Methodology. A face-centered cube design was adopted for this study, and the optimal values for these factors were predicted by the resulting experimental equations. The degradation of toluene by P. fluorescens was tested by incorporating different concentrations (0.02, 0.04, 0.06, 0.08 and 0.10%) of humic acids, extracted from peat moss, in a hydrocarbon degradation medium. The results showed that the microbial degradation of toluene was inhibited by the presence of humic acids. (C) 2003 Elsevier Science Ltd. All rights reserved.