A rigorous steady state model of acetate-utilizing methanogenic biofilms is developed accounting for the mass transfer of neutral and ionic species, pH changes within the biofilm, pH-dependent Monod kinetics, chemical equilibrium, electroneutrality, gas production within the biofilm, and the presence of a concentration boundary layer (CBL). In contrast to traditional biofilm models where the pH is assumed to be constant within the biofilm, an increase in pH in acetate-utilizing methanogenic biofilms is predicted. Furthermore, significant differences can exist between the flux predictions using the traditional models and when pH changes within the biofilm are taken into account. The optimum pH for acetate-utilizing biofilms is less than the optimum defined for suspended-growth systems. The biofilm model is coupled to a model of a completely-stirred tank reactor (CSTR), and strategies for the optimization of biofilm reactor performance are examined. For a fixed set of operating conditions, an optimum influent pH can be defined that corresponds to the maximum removal efficiency and flux of acetate into the biofilm.