A brief overview of the existing chemical and biological models of biodegradation in porous media is given. A revised biological model of aerobic degradation of organic matter in benthic sediments is developed. The model is based on steady state balance equations for microbial mass, organic matter, and oxygen. The biomass balance equation involves a logistic trophic function (instead of the classical Malthusian one) describing the self-regulation of microbial abundance. One of the corollaries of the model is the dependence of the decay rate upon the organic matter content S in the form dS/dt∝S2/(S+KS)2 (where KS is a constant) that implies two extreme cases: dS/dt is independent of S at small KS (refractory organics) and dS/dt∝S2 at large KS (labile organics), instead of the commonly used law dS/dt∝S. The oxygen consumption rate J is expressed through the oxygen concentration O0 in the overlying water and three generalized parameters. This function demonstrates the effect of the plateau at large O0. The dependence J(O0) is calibrated for the three experimental systems of the literature, two of which have high oxygen consumption rates and one low. For the latter, the model parameters are estimated and their correspondence to the evidence available is discussed. Depth profiles of oxygen, organic matter, and biomass are computed and the aerobic zone thickness is determined. ¿ 2000 American Geophysical Union