Abstract The inhibitory effect of ammonium sulfate on microorganism growth has been studied for a commercial mixed culture utilized in biological wastewater treatment under aerobic batch conditions. Andrews' mathematical expression was then applied to fit the kinetic pattern of ammonium sulfate inhibition on the specific growth rate of the culture. Having Andrews' kinetics, a mathematical model describing the change in effectiveness factor for inhibitory substrate removal in a spherical biomass was developed. Typical results for substrate concentration profiles and unusual results for effectiveness factors from the numerical solution of the resultant dimensionless equation were presented. Higher bulk substrate concentrations caused effectiveness factor to be more than unity due to dominating inhibition at the surface of the biomass particle. However, in low bulk concentrations when the inhibition mechanism is not dominant, classic behavior of the effectiveness factor was observed.