Three-dimensional field monitoring of a gasoline plume showed rapid decay of toluene and ethylbenzene during downgradient transport with slower decay of xylenes, benzene, and MTBE under mixed aerobic-denitrifying conditions. Decay was most rapid near the source but slower farther downgradient. Effective first-order decay coefficients varied from 0 to 0.0010 d⁻¹ for MTBE, from 0.0006 to 0.0014 d⁻¹ for benzene, from 0.0005 to 0.0063 d⁻¹ for toluene, from 0.0008 to 0.0058 d⁻¹for ethylbenzene, from 0.0012 to 0.0035 d⁻¹ for m-, p-xylene, and from 0.0007 to 0.0017 d⁻¹ for o-xylene. Laboratory microcosm studies confirmed MTBE biodegradation under aerobic conditions; however, the extent of biodegradation was limited.