Biofilm reactors are an emerging environmental biotechnology used for the removal of selenium from industrial wastewater. Bioreactor models for selenium removal that are based on the mechanistic description of biofilms are few in number, but a dynamic model that is based on mechanistic principles (chemical and biochemical) and is capable of describing selenium removal is needed for efficient and more rigorous process design and optimization. This paper summarizes variables, processes, and rate expressions for both chemical and biochemical conversions inside a mixed-culture biofilm containing facultative heterotrophs, sulfate reducing bacteria, and autotrophic denitrifiers. Chemical and biochemical processes in the biofilm model are based on theoretical considerations. Chemical processes include acid-base equilibria reactions primarily to describe substrate. The proposed mathematical description of our multispecies selenium removal biofilm is based on the general one-dimensional model of Wanner and Gujer (1985) and Wanner and Reichert (1995). It is assumed that ions do not undergo chemical or biological transformation, but the ions do not maintain a constant concentration profile across the one-dimensional biofilm. State variables, processes, and kinetic expressions in the biofilm model are structured using (Peterson) matrix notation. The framework for a mechanistic mathematical model describing a dynamic one-dimensional biofilm has been developed and presented.