A new method is demonstrated for calculating gas adsorption heats from variable-temperature infrared spectra of adsorbed molecules. By focusing on the low coverage regime, where (at equilibrium) there is only a very small fraction of molecules adsorbed, the procedure devised yields the corresponding value of adsorption enthalpy without needing to know the equilibrium pressure. Experimental measurements can thus be highly simplified, which constitutes one of the merits of the new method. Versatility for dealing with adsorption processes occurring at a low temperature, and having a small energy balance, is another favourable feature.