Cluster model DFT calcns. of CO adsorption on various possible forms of gallium in Ga/HZSM-5 zeolites have been performed. CO was found to only weakly interact with Ga+, (GaO)+, and (Ga(OH)2-nHn)+ (n = 1, 2) cationic clusters. The resulting shifts of the CO stretching frequency (.nu.CO) are only very small. On the other hand, CO coordination to small mononuclear and binuclear Ga3+ hydride/hydroxide/oxide species results in pos. shifts in the stretching frequency in the range .nu.CO = 20-40 cm-1. Larger shifts (.nu.CO = 70-90 cm-1) are assocd. with CO coordination to Ga ions at the corners of small three-dimensional Ga-oxide clusters. The exptl. obsd. changes in the IR spectrum of adsorbed CO over Ga/HZSM-5 zeolites upon reductive and oxidative treatments are interpreted with these insights. Possibilities for the formation of such polynuclear oxide species in the zeolite micropore space are discussed. On the basis of recent literature insights, we suggest that large shifts derive from CO coordination to oligomeric Ga cationic complexes stabilized by the neg. zeolite charge.