A framework for numerically modeling the biodegradation of petroleum hydrocarbons within a flooded column of porous media in which a residual saturation of petroleum is present is developed. The petroleum product of interest is considered to be representable, at a desired level of detail, as a mixture of a finite number of classes of hydrocarbon components. Of these classes, all but one are assumed to be biodegradable by indigenous microorganisms. A specific microbial population is associated with each degradable class. Microbes growing at the oil-water interface take up the hydrocarbon components of the petroleum through the use of extracellular pseudosolubilization factors, which facilitate transport from the oil-water interface into the cells. Minimum Monod kinetics is used to model many of the biological processes. The transport of dissolved oxygen, nitrogen, and phosphorus through the column provides the microorganisms with the required electron acceptor and nutrients for aerobic metabolism. ¿ American Geophysical Union 1994