The role of soil, straw, and sawdust as supports in enhancing pentachlorophenol (PCP) mineralization by an indigenous soil consortium was examined by assessing the bioavailability of the substrate and other nutrients. PCP sorption tests were conducted in the presence of sterile supports to evaluate PCP bioavailability. Indigenous biomass, practically free of soil particles, was prepared to test the influence of sterile soil and soil components on the mineralization of increasing PCP concentrations. Organic supports such as straw and sawdust were very good sorbents for PCP, resulting in a slow, continuous desorption of substrate, high mineralization rates, and reduced toxicity to the active biomass. Soil and clay retained less PCP and desorbed it in decreasing amounts. Soil was the best amendment to enhance the mineralization of 100 mg/L PCP. Soil, soil extract, and the lowest-molecular-weight fraction of the soil extract facilitated the complete mineralization of 300 mg/L of PCP with a lag time of about 9 days, compared to 21 days for the unamended culture. Addition of soil enhanced PCP mineralization by an indigenous consortium, probably because soil particles served as an adsorbent for the contaminant to decrease its toxicity, as a support for biomass colonization, and as a source of supplementary nutrients for the biomass. This concept is of importance, particularly for the production of active and resistant biomass for the biotreatment of contaminated soils.