Adsorption of carbon monoxide has been studied on Zeolite Y in which Na+ ions have been partially exchanged for Zn2+, Mn2+, Co2+, Ni2+, Cu2+, Ba2+ UO2+2 and Ce3+ ions. The divalent ions all produced sites for specific CO adsorption, even at low degrees of exchange. Thus, unlike Ca2+ studied in earlier work, these ions do not have a total preference for internal sites in the prisms or sodalite units inaccessible to CO. At approximately 30% exchange the affinity of divalent ions for internal sites decreases in the order Ca2+ > Ni2+, Mn2+, UO2+2 > Cu2+ > Zn2+. However, even on the Zn-exchanged Y, only about 1 divalent ion in 10 acts as a site for specific adsorption of CO. Very little CO was specifically adsorbed on Ce-exchanged Y, even at high degrees of exchange. This confirms that cerium ions favour internal sites. With ZnY, MnY, BaY and CeY, CO adsorption was rapid and reversible. Isosteric heats were evaluated and, except for CeY, were substantially greater than on unexchanged NaY, confirming the presence of specific adsorption. The heats correlate with the electrostatic field strengths of the cations and with the shifts of the C—O stretching frequency on adsorption. With NiY and CuY, adsorption was slow and is thought to reflect a gradual increase in the number of adsorption sites caused by adsorbate-induced migration of divalent cations. A number of the systems studied in this work have not previously been investigated from the standpoint of cation location. However, where comparisons are possible, the present results derived from CO adsorption are shown to be in good agreement with those from other methods.