Within the MINDO/3 method, cluster models giving a description of highly dispersed magnesium chloride containing all sets of acid sites, i.e. three-, four- and five-coordinate magnesium ions, and surface hydroxyl groups have been proposed. Three types of CO adsorption complexes on the magnesium chloride surface have been considered. The C-O stretching frequencies of these adsorption complexes and the O-H stretching frequencies of hydroxyl groups on magnesium chloride calculated by a harmonic oscillator approach are in good agreement with available experimental and theoretical data.