The effect of compound structure on the microbial transformation of a series of substituted anilines was investigated. For the pure-culture and environmental water samples studied, the rate of transformation of the compounds decreased in the following order: aniline, 3-bromoaniline, 3-chloroaniline, 3-methylaniline, 3-methoxyaniline, 3-nitroaniline, 3-cyanoaniline. Second-order rate constants (kb) for each compound was calculated by using bacterial and compound concentration measured as a function of time. The rate constants correlated with steric parameters. Water samples also were used in kinetic studies with three of the compounds (aniline, 3-chloaniline, and 3-nitroaniline) to test the relationships with mixed bacterial populations. A simple linear regression of van der Waals radius of the substituent group with log kb gave correlation coefficients (r2) of 0.924 for the river isolate and 0.99 for the mixed populations. Analyses of pure-culture and mixed population samples by thin-layer chromatography indicate that the primary products are catechols. The finding suggests that the transformation pathway involves oxidative deamination of the anilines.