Hydrogen sulfide formation is suppressed by a three-way conversion catalyst having an underlayer and a topcoat overlying the under layer. The under layer is prepared by dispersing a Group IIa metal oxide such as an oxide of magnesium, calcium, barium or strontium on a carrier such as a refractory metal oxide monolith. A topcoat overlying the undercoat is comprised of a three-way conversion catalyst material such as a platinum-group metal catalytic component, e.g., platinum, palladium, rhodium or mixtures thereof.