Naphthenic acids are a complex mixture of organic acid compounds which naturally occur in crude oil. The naphthenic acids are corrosive to equipment during the extraction process from oil sands. They become concentrated in tailings pond water that is retained at the oil sands mining sites in northeastern Alberta. Environmental regulations require tailings water to be retained in large ponds to prevent release into the environment due to concerns about toxicity. A major source of toxicity in tailings pond water has been linked to naphthenic acids. While research to date indicates that natural biodegradations processes do decrease the acute toxicity of naphthenic acid compounds, the kinetics of the degradation and their rates are not clearly understood. Also, as a result of increases in capacity of the oil sand processing plants, improved and more timely alternatives for bioremediation of naphthenic acids are required. In this study, a model naphthenic acid compound (trans isomer of 4-methyl-1-cyclohexane carboxylic acid) and a microbial culture, developed in our laboratory, have been used to evaluate the kinetics of biodegradation in batch and continuous bioreactors. The effects of naphthenic acid concentration on the kinetics of microbial growth are presented. A comparison of the effects of two bioreactor configurations is demonstrated.