Commercially-prepared Pt-Rh and Pd monolith catalysts were thermally aged then characterized for catalytic performance using a laboratory reactor to evaluate the magnitude and reversibility of the impact of sulfur on three-way activity. The SO2 concentration in the feedstream was varied from 0 ppm to 30 ppm, which was comparable to sulfur levels in gasoline ranging from 0 to 450 ppm. Tests were first conducted using propylene and repeated using propane to represent the hydrocarbon mixture in exhaust. Of the two catalysts, Pd showed better propylene lightoff activity while Pt-Rh showed better propane lightoff activity, regardless of the sulfur content. For each catalyst, increasing the sulfur concentration from 0 ppm SO2 to 30 ppm SO2 of sulfur resulted in a lightoff temperature increase by 40 to 60~ Under warmed-up conditions, the loss of activity for HC, CO and NOx due to the presence of sulfur was greater under slightly rich conditions than under lean conditions for both Pd and Pt-Rh, while the magnitude of the impact on NOx and particularly on HC activity under warmed-up stoichiometric conditions was significant and much greater for Pd than for Pt-Rh catalyst. Using propylene, the effect of SO2 on the activity of the Pd catalyst was partly reversible, while the effect on Pt-Rh was completely reversible. Using propane, the effect of sulfur on the activity of both catalysts was larger than for propylene. The resulting decrease in activity due to the presence of sulfur was partly reversible on Pt-Rh, but the poisoning of the Pd catalyst was mostly irreversible. Part of the irreversible poisoning effect is attributed to a direct interaction or reaction between SO2 and Pd, while the other part is attributed to the promotional effect of SO2 in hydrocarbon coking of the catalyst when alkane hydrocarbons are present.