Materials formed by the incorporation of rare earth cations (M = La, Pr, Nd, Sm, Gd) into tin(IV) oxide using coprecipitation methods show no significant enhancement of catalytic activity toward the oxidation of carbon monoxide or propane over that of tin(IV) oxide itself. For chromium-promoted tin(IV) oxide catalysts, the temperature by which complete conversion of carbon monoxide and propane occurs is dependent on both the Cr:Sn atom ratio in the catalyst and the preparative route by which the chromium is incorporated into the catalyst. As prepared all the materials are hydrous gels comprising very small (<10 nm) particles of tin(IV) oxide over which the modifying metal component appears to be dispersed uniformly. Chromium(VI) oxyanions of the types CrO42-, Cr2O72-, and Cr3O102- are sorbed on to the surface of the tin(IV) oxide particles in the freshly prepared material derived from aqueous CrO3 and tin(IV) oxide gel. Prior to calcination the materials are microporous, but significant changes in specific surface area, pore volume, and pore size occur at temperature >673 K. Powder X-ray diffraction and electron microscopy confirm the formation of Cr2O3 on calcination at 1273 K.