The state of the chromium ions on Cr/ZrO2 and Cr/TiO2 samples has been tested by the IR spectra of adsorbed CO and NO. CO adsorption on activated samples leads mainly to formation of carbonyls related to the support. At low temperature, CO is also bonded to Crn+-OH groups that are present in low concentration. When NO is adsorbed on these samples, it forms Crn+-NO (n > 3) species and is slowly oxidized to NO+, NO2-, and NO3- Small amounts of Cr (3+)(NO)(2) species are also detected, which evidences the existence of a limited fraction of Cr3+ cations. Reduction of both samples leads to conversion of all chromium into Cr3+ cations. With CO, they form relatively stable linear carbonyls whereas dinitrosyls are mainly formed after NO adsorption. In addition, a fraction of Cr3+ -OH groups with a very weak acidity is formed by reduction. CO-NO coadsorption experiments have revealed that NO is selectively adsorbed on Cr3+ sites, whereas CO is preferentially located on Zr4+ or Ti4+ cations. The possibility of selective detection of chromium sites on supported chromia catalysts is discussed.