Developing an improved understanding of enhanced biodegradation is of great importance in remediation of contaminated soils and aquifers and in cleanup of oil spills. The effect of several nonionic and anionic surfactants and their mixtures on the biodegradation of n-decane was investigated. Microbial growth of Pseudomonas aeruginosa on the solubilized hydrocarbon was found to be stimulated by all of the non-ionic surfactants tested, with varying degrees of enhancements in the rate of biodegradation compared to that found in the absence of surfactant. Three anionic surfactants, Linear Alkyl benzene Sulfonate (LAS), Sodium Dodecyl Sulfate (SDS) and Neodol 25-3S, an alcohol ethoxysulfate, were investigated. Both SDS and N25-3S stimulated the degradation of n-decane. LAS did not support the growth of P. aeruginosa with or without decane but its mixtures with the nonionic surfactant Neodol 25-7 did enhance the overall degradation except at very high LAS contents. A novel aspect of this study was the simultaneous measurements of surfactant concentration, cell mass, and n-decane concentration as a function of time when the surfactant was SDS. A modified Monod model was developed to simulate bacterial growth rate and substrate and surfactant degradation. Predictions of the model were in good agreement with experimental data. With the assumption that the cell mass generated by degradation of a given quantity of n-decane was the same when other surfactants were used, it was possible to apply the model successfully in other systems where it was not feasible to measure surfactant degradation. The half saturation constant K\sbS for n-decane degradation was high both for highly degradable surfactants and for poorly degradable mixtures such as those with high contents of LAS. A minimum value of K\sbS is reached for surfactants and surfactant mixtures with intermediate degradation rates. The degradation rate of n-decane was maximized for this condition.