The interactions of carbon monoxide with Lewis acid sites of zeolites were investigated using microcalorimetric and temperature programmed desorption (TPD) techniques. Carbon monoxide was adsorbed at 25 degreesC on transition-metal ion-exchanged FAU (Y and X) and MOR type zeolites. The differential heats of adsorption (Q(dif)) indicate the heterogeneity of the acid sites, in all the investigated systems. The specific interactions of CO with the strongest acid sites were recognized. The results of TPD confirm that CO molecules were adsorbed on the sites of different strength.