Uranium oxide catalysts supported upon gamma-Al2O3 and amorphous and mesoporous silica have been tested for CO/O-2 and NO/CO conversions. Pre-calcination is deleterious to catalysts on a mesoporous support due to extrusion of the active uranium phase into poorly dispersed uranium oxide particles. Developing the catalysts under CO/O-2/He or CO/NO/He avoids extrusion and results in superior mesoporous catalysts (from uranyl nitrate precursor) which are comparable to Pt/Al2O3 catalysts. Chloride is found generally to be deleterious to catalyst performance. The exception to this in UCl4/gamma-Al2O3 calcined at 873 K for CO/O-2. In this system, the total activity is correlated to the level of chlorine retention and average uranium phase crystallite size within the catalyst. The net rate expression for selective reduction of NO by CO over the best mesoporous catalyst is zero order in NO and proportional to [CO](1,4) in contrast to bulk U3O8 where the net rate depends upon [NO]. The activation energy (E(,)) and pre-exponential factors (v) are highly correlated both to each other and the net dispersion of the UO2.2 phase recovered after catalysis; the most active systems are highly dispersed where the E(,) values are more than compensated for by higher values of v. (c) 2005 Elsevier B.V. All rights reserved.