First published as an Advance Article on the web 23rd October 2002 Variable-temperature FTIR spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure, is shown to be a convenient method for the thermodynamic study of adsor-bent–adsorbate systems. When weak interactions are concerned, the technique presents favourable features, as compared to clas-sical microcalorimetric measurements. This recently developed spectroscopic method is demonstrated by studying the adsorption of dinitrogen on the protonic zeolite H-ZSM-5, a system for which the availability of microcalorimetric measurements affords a direct check of the new method. The relevant thermodynamic quantities determined for this system are DH ¼ À19.7(AE0.5) kJ mol À1 and DS ¼ À125(AE5) J mol À1 K À1 ; the standard adsorption enthalpy compares favourably with the microcalori-metrically determined value of about 19 kJ mol À1 . Two classical methods for determining heats of adsorption are in current use: adsorption microcalorimetry and the measure-ment of adsorption isotherms over a temperature range. Basi-cally, microcalorimetry (combined with suitable gas dosing equipment) yields differential heats as a function of coverage, from which an average integral heat can be derived. Alterna-tively, application of the Clausius–Clapeyron equation to a series of adsorption isotherms obtained at different tempera-tures yields the so called isosteric heat (or enthalpy) of adsorp-tion. We have recently found that, in favourable cases, variable-temperature IR spectroscopy can be advantageously applied. This new procedure involves the use of an IR cell which allows IR spectra to be recorded over a wide tempera-ture range while simultaneously measuring temperature and equilibrium pressure. The principle of the method is as follows. Consider the adsorption process described by eqn. (1) below, where S is the adsorption site at a solid (adsorbent) surface and M is a molecule being adsorbed from the gas phase: S ðsÞ þ M ðgÞ Ð S M ðadsÞ ð1Þ