Zeolites are often used as the host material for holding and organizing adsorbed molecules and supramolecular species inside their void channels and cages, in order to exploit space confinement and host-guest interaction for engineering composite materials having novel electronic and optical properties. That endeavour would benefit from improved knowledge about the type and strength of the zeolite adsorption sites. To this end, variable temperature infrared spectroscopy (a technique capable of giving simultaneous information on the type and strength of gas-adsorption complexes) was used herein to characterize the zeolite K-L by means of adsorbed CO. Two types of cationic adsorption sites (termed D' and D '') were found on the wall of the zeolite main channel; formation of the corresponding CO adsorption complexes was found to involve a standard enthalpy change of -23.4 and -26.7 kJ mol(-1), respectively. (C) 2015 Elsevier B.V. All rights reserved.