Abstract: Three-way precious metal catalysts useful for conversion treatment of pollutants in fuel combustion gases from automotive exhausts, or gas emissions from stationary fuel combustion sources, have chemical activity and thermal stability of the catalysts appreciably enhanced by utilizing a composite oxygen-ion conducting support material. Such composite supports are composed of zirconia stabilized by yttria, calcia, magnesia or scandia, and at least 40 wt. % inert support material such as alumina or titania, and have surface area of 20-300 m.sup.2 /gm. The catalysts can be supported by composites of yttria-stabilized-zirconia (YSZ) and alumina, and contain 0.01-3.0 wt % total active metals, which may consist of major metals platinum or palladium and minor metals rhodium or ruthenium dispersed on the support. Such three-way catalysts are thermally stable, achieve a high level of pollutants (CO, NO.sub.x, H.sub.