Transition metal salts were added to aerogels of TiO2-SiO2 mixtures. The resulting M-TiO2-SiO2 (M = transition metal, in general present in the form of oxides) materials had very high surface areas (600-1000 m(2) /g) and pore volumes (1-4 cm(3)/g), and contained highly dispersed transition metal oxides. Several samples were active photocatalysts for acetaldehyde oxidation to carbon dioxide using visible light; in particular chromium, cobalt, and nickel. These intimate aerogel mixtures were only active when SiO2 was present (i.e. M-TiO2 aerogels were not). High oxidation states of the transition metal ions (e.g. Cr6+, Co3+) appear to be important in bringing about visible light photocatalyst activity. (C) 2003 Elsevier B.V. All rights reserved.