Oxidation of carbon monoxide was studied on Mg- and Al-substituted CuCr2O4 spinel catalyst at atmospheric pressure and temperatures between 373 and 723 K. The activity of CuCr2O4 decreased even for small replacements of either Cu by Mg or Cr by Al and none of the substituted oxides was as axtive as CuCr2O4. In Cu1-xMgxCr2O4 catalysts, the activity systematically decreased with increasing x, except for 0.4 < x < 0.6. The decrease in activity is due to a decrease in the active Cu2+ ions of the catalyst. The increase in activity on increasing x from 0.4 to 0.6 is attributed to the crystallographic phase change, i.e., tetragonal to cubic, in the catalyst. This was also found in the CuCr2-xAlxO4 catalysts. The decrease in the catalytic activity on substitution of Cr by Al, even when the total copper content is not altered, is due to the reduction of some of the active Cu2+ ions to Cu1+ ions.