Chromium(III) acetate impregnated onto a high surface area silica support and chromium(III) acetylacetonate dry-blended with silica were calcined at 1053 K and cooled. CO reduction of the resultant CrOx/silica materials was monitored by quadrupole mass spectrometry and found to proceed via a Langmuir-Hinshelwood mechanism. Activation energies for CrOx/silica catalyst reduction by CO have been determined from the corresponding Arrhenius plots; these suggest the existence of two types of reactive chromium sites.