CD3CN is used for the first time for characterizing Lewis sites of MgO ex-hydroxide in a combined IR and ab initio study. Four v(CN) bands of coordinated CD3CN species are observed at 2317, 2302, 2293, and 2280 cm(-1) at low coverages. The 2302 and 2293 cm(-1) bands are attributed to CD3CN on Mg-3c and Mg-4c sites at the intersections of regular MgO faces of the (001) type. The 2317 and 2280 cm(-1) bands are assigned to CD3CN stabilized by two neighboring Mg-4c sites on (110) defect faces and by two neighboring Mg-3c sites at the intersections of the (001) and (110) faces, respectively. This assignment is supported by the theoretical reproduction of the experimental Delta v(CN) shifts to an accuracy of a few cm(-1). The only other detected v(CN) band, which appears at high coverages, is the v(CN) = 2261 cm(-1) band of liquid CD3CN. This fact and results of frequency and energy analyses suggest the energy of CD3CN adsorption on Mg-5c sites of the (001) faces to be smaller than the energy of CD3CN liquefaction.