The transport and biodegradation of an organic compound (quinoline) were studied in a meter-scale system of layered porous media. A two-dimensional laboratory experiment was conducted in a saturated system with two hydraulic layers with a ratio of conductivities of 1:13. A solution containing dissolved quinoline was injected as a front at one end of the system, and the aqueous-phase concentrations of quinoline, its first degradation product (2-hydroxyquinoline), and oxygen were monitored over time at several spatial locations. Results from a set of ancillary batch and small-column experiments were used to generate a mathematical model for the microbial kinetics; these kinetics described the time rate of change of the concentrations of the two organic compounds (quinoline and 2-hydroxyquinoline), the electron acceptor (oxygen), and microbial biomass. This independently developed kinetic model was incorporated into a two-dimensional numerical model for flow and transport, so that simulations of the laboratory system could be conducted and the results compared with observed data. An analysis of the applicability of single-phase and multiple-phase models for the description of the microbial kinetics was conducted. The results of this analysis indicated that for some cases, it is not necessary to explicitly model the mass transfer between the aqueous phase and the biomass phase. A single-phase model was used for simulating the laboratory system described here. Favorable comparisons between the laboratory and simulation data suggested that a single-phase model was appropriate for describing the microbially mediated reactions in this system. A method for incorporating the effects of metabolic lag into microbial kinetics is described. Metabolic lag was explicitly accounted for in the degradation kinetics for this system; the inclusion of metabolic lag proved to be important for describing transient concentration pulses that were observed in the low-conductivity layer. ¿ American Geophysical Union 1994