This paper demonstrates the application of the factorial sensitivity analysis methodology in studying the influence of variations in stoichiometric, kinetic and operating parameters on the performance indices of an oxidation ditch simulation model (benchmark). Factorial sensitivity analysis investigates the sensitivities in a region rather than in a point. Hence, it has the advantage of giving more information about parameter interactions (non-linearity). Short-term results obtained have shown the following. The index AE is not significantly affected by variations in the value of parameters of the activated sludge model (ASM) No 1. The index TSP is greatly influence by heterotrophic yield (Y-H), heterotrophic decay (b(H)) and specific hydrolysis (k(h)) and the index EQ is dominated by Y-H, Monod coefficient (K-S), b(H), k(h), anoxic condition correction factors (eta (g), eta (h)), hydrolysis half-saturation coefficient (K-X), autotrophs maximum specific growth rate (mu (A)) and ammonia half-saturation coefficient (K-NH) Furthermore, the index EQ has been shown to be very sensitive to parameter interactions, at certain regions.