Particulate catalysts, consisting of porous solid particles, have been under intensive development for the control of automotive emissions. Such catalysts contain base metal and/or noble metal active components dispersed on high surface area alumina supports. Consideration is given to the physical and chemical properties of both supports and catalysts, and to the relationship between some of these properties and catalytic performance. It has been found that low bulk density and the presence of macroporosity improve conversion efficiency. At high temperatures, transformation of the support from a high surface area alumina to crystalline alpha alumina causes shrinkage, a deterioration of mechanical properties, and catalyst deactivation. Support interactions with certain activity promoters accelerate this adverse transformation. Stabilizers have been found that repress the alpha alumina transformation. Base metal catalysts are extremely sensitive to sulfur poisoning. Resistance to sulfur poisoning can be increased substantially by promotion of base metal catalysts with noble metals.