Strategies to enhance biotransformation of pentachlorophenol (PCP) in a spectrum of wetland soils were investigated under laboratory conditions, which included manipulations of electron acceptors and donors, and PCP concentrations. Maximum transformation rates were found at PCP concentrations <10 µM (methanogenic conditions) and >6 µM to >23 µM (aerobic conditions). Differences in PCP toxicity and sorption among soils and treatments were largely governed by the activities of microbial groups. Within this concentration range, transformation was observed in soils under aerobic and methanogenic conditions, but was inhibited under denitrifying and SO₄^2--reducing conditions. Aerobic PCP transformation initially produced small amounts of pentachloroanisole (PCA). However >75% of both chemicals disappeared in 30 d from five soils. Measured soil properties were not significantly correlated to aerobic transformation rates. Under methanogenic conditions, PCP was reductively dechlorinated to yield a mixture of tetra-, tri-, and dichlorophenols in eight soils, with rates strongly correlated to measures of electron donor supply (total C, N, organic C mineralization rates) and microbial biomass. Addition of protein-based electron donors enhanced reductive dechlorination in a soil low in organic matter and microbial biomass. Results demonstrated the widespread occurrence of PCP transforming microorganisms in soils, which may be promoted by manipulating environmental conditions.