This is the third and final communication in this series of laboratory evaluation of three-way catalysts. The effect of inlet NO concentration and temperature on the NH3 formation over fresh, pulsator-aged and dynamometer-aged three-way catalysts of the current generation has been investigated under temperatures and exhaust compositions of practical interest. In spite of differences in aging procedures employed, both the pulsator and dynamometer-aged catalysts show similar selectivity behavior. The effect of SO2 in feed-gas on gross NO conversion and NH3 formation was studied over Pt-Rh and Pt-Rh-Ru types of three-way catalysts. A strong dependence of the gross NO conversion on the SO2 concentration in exhaust gas mixtures was noted. A simultaneous suppression of gross NO conversion and NH3 formation, in presence of SO2 in feed-gas, is attributed to the poisoning of Pt sites on aged three-way catalysts. IR studies of SO2 and CO adsorption over Pt, Rh and Ru metals suggest that differences in the surface affinity of these metals to SO2 can be related to their observed activity patterns. Finally, reoxidation of NH3 (formed over three-way catalysts) on the vehicle-aged oxidation catalyst was studied as a function of catalyst temperature, excess oxygen and inlet NH3/NO ratio.