A model has been developed describing the microbial reduction of solid-phase electron acceptors (Fe(III) oxides) as well as dissolved electron acceptors (chelated Fe(III) or organic electron shuttles) in Shewanella alga BrY. The model utilized a multiple-substrate, Monod kinetics formulation. The Monod description of solid Fe(III) reduction requires a normalization of surface Fe concentration to biomass concentration in order to describe the bioavailable Fe(III) concentration. The model also contains provisions for irreversible sorption of Fe(II) to Fe(III) oxide surfaces and for the precipitation of Fe(III) carbonates. The loss of bioavailable Fe(III) due to sorption of Fe(II) was found to be minor, even for highly sorptive amorphous Fe(III) oxyhydroxides. However, the final extent of microbial reduction is very sensitive to the rate of siderite precipitation, assuming that siderite precipitation could partially occlude Fe(III) surface sites. The use of a multisubstrate Monod kinetics model enabled an evaluation of the effects of electron shuttles on solid Fe(III) reduction. Because the electron shuttle is recycled, very small additions can greatly increase the overall rate of solid Fe(III) reduction.