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 adsorbent-adsorbate systems. When weak interactions are concerned, the technique presents favourable features, as compared to classical 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 ΔH° = -19.7(±0.5) kJ mol^-1 and ΔS° = -125(±5) J mol^-1 K^-1; the standard adsorption enthalpy compares favourably with the microcalorimetrically determined value of about 19 kJ mol^-1.