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.