A comprehensive experimental evaluation of tannery wastewater was performed as a basis for the modeling of the activated-sludge process. Characterization studies involved, aside from conventional parameters, a detailed chemical oxygen demand (COD) fractionation for the identification of biodegradable and inert COD components. Respirometric analysis was carried out to assess significant rate constants associated with the stoichiometry and kinetics of activated sludge. Specific emphasis was placed on the evaluation of the hydrolysis mechanism for slowly biodegradable GOD, apparently the rate-limiting step for organic carbon removal. Parallel experiments were run both on plain and chemically settled samples to investigate the effect of chemical settling on biological treatability. The fate and variation of primary process components affecting effluent quality with the sludge age were evaluated by means of model simulations using recent multicomponent modeling tools and generated experimental data.