First published as an Advance Article on the web 3rd June 2003 By combining FTIR spectroscopic and microcalorimetric measurements on adsorbed CO, two lithium sites were found to be present in the zeolite Li-ZSM-5. Both of these sites, Li A and Li B , form Li(CO) + monocarbonyl species with adsorbed CO; the corresponding C–O stretching frequencies are 2193 and 2187 cm À1 , respectively. Formation of Li A (CO) + involves an estimated adsorption enthalpy of DH ffi À 44 kJ mol À1 , while for Li B (CO) + the corresponding value is evaluated to be DH ffi À18 kJ mol À1 . The large difference between these DH values is explained in terms of an endothermic process occurring during formation of the Li B (CO) + species, which involves a slight movement of the corresponding Li + ion (away from its equilibrium position among oxygen anions of the zeolite framework). Formation of dicarbonyls is also discussed, as well as formation of O-bonded Li(OC) + species.