Cesium-exchanged zeolite X was impregnated with cesium acetate (Cs(Ac)/CsX) or cesium carbonate (Cs(2)CO(3)/CsX) and subsequently calcined to yield a basic catalyst. The Raman spectra of calcined Cs(Ac)/CsX and Cs(2)CO(3)/CsX exhibited a new peak at 1036 cm(-1) associated with the occluded species. No evidence for cesium peroxide or superoxide was observed. The occluded cesium species in both samples is proposed to be an oxycarbonate, which is a metastable intermediate between cesium carbonate and cesium oxide. The isomerization of 1-butene to cis- and trans-2-butene was catalyzed by cesium-loaded zeolite X. Although CO(2) readily poisoned the active base sites for catalysis, pretreatment of a basic zeolite with O(2) at 373 K did not. Co-feeding O(2) with 1-butene at 373 K, however, completely deactivated the base sites. Analysis of the reactor effluent at 473 K and the IR spectrum of the catalyst indicated the formation of carbon dioxide, which irreversibly adsorbed on the basic sites of the catalyst. Deactivation of basic catalysts by O(2) is proposed to occur through a low temperature oxidation of 1-butene to carbon dioxide, which strongly adsorbs on the active sites.