Bronsted acidity is a key factor controlling catalytic performance of zeolites in many technological processes, and hence the need to find a reliable means for measuring acidity strength. The enthalpy change, Delta H-0, involved in the hydrogen bonding interaction between a weak base, such as carbon monoxide or dinitrogen, and the Bronsted acid [Si(OH)Al] hydroxyl groups should correlate with the zeolite acid strength. However, on account of simplicity, the red shift of the O-H stretching frequency, Delta nu(OH), in the hydrogen-bonded OH center dot center dot center dot CO or OH center dot center dot center dot N-2 complexes is frequently measured (instead of Delta H-0) and correlated with acid strength; and in fact it is often found that Delta H-0 and Delta nu(OH), correlate among themselves, but this might not always be the case. We report herein on a variable-temperature IR (VTIR) spectroscopic study of the interaction of CO and N-2 with the protonic zeolite H-MCM-22 (structure type MWW). From series of infrared spectra taken over a temperature range, the standard enthalpy of formation of the OH center dot center dot center dot CO or OH center dot center dot center dot N-2 complexes was found to be -23.5(+/- 2) and -14.5(+/- 2) kJ mol(-1), respectively. Corresponding A voH values are -321 and -123 cm(-1), respectively. Comparison with corresponding Delta H-0 and Delta nu(OH) values reported in the literature for other Bronsted-acid zeolites shows strong evidence of a limited validity of the rule correlating Delta nu(OH) with Delta H-0 for both probe molecules, which gives a clear warning about the risk of using the magnitude of A voH as a measure of zeolite acidity. (C) 2013 Elsevier B.V. All rights reserved.