Elemental sulfur (S-0) can serve as an electron donor for water and wastewater denitrification, but few researchers have addressed the kinetics of S-0-based reduction of nitrate (NO3-), nitrite (NO2-), and nitrous oxide (N2O). In addition, S-0-based denitrifying biofilms are counter-diffusional. This is because the electron donor (S-0) is supplied from the biofilm attachment surface while the acceptor, for example, NO3-, is supplied from the bulk liquid. No existing mathematical model for S-0-based denitrification considers this behavior. In this study, batch tests were used to determine the kinetic parameters for the reduction of NO3-, NO2-, and N2O. Additionally, a biofilm model was developed to explore the effects of counter-diffusion on overall fluxes, that is, the mass of NO3- or NO2- removed per unit biofilm support area per unit time. The maximum specific substrate utilization rates (q) for NO3-, NO2-, and N2O were 3.54, 1.98, and 6.28 g N g COD-1 center dot d(-1), respectively. The maximum specific growth rates (mu) were 0.71, 1.21, and 1.67 d(-1) for NO3- to NO2-, NO2- to N2O, and N2O to N-2, respectively. Results suggest that the observed NO2- accumulation during S-0-based denitrification results from a low q for NO2- relative to that for NO3-. The high q for N2O, relative to that for NO3- and NO2-, suggest that little N2O accumulation occurs during denitrification. A counter-diffusional biofilm model was used to predict trends for NO3- fluxes, and confirmed NO2- accumulation in S-0-based denitrification biofilms. It also explains the observed detrimental effects of biofilm thickness on denitrification fluxes. This study allows a more accurate prediction of NO3-, NO2-, and N2O transformations in S-0-based denitrification.