Adsorption of dihydrogen onto the zeolites Na-ZSM-5 and K-ZSM-5 renders the fundamental H-H stretching mode infrared active. The corresponding infrared absorption bands were found at 4101 and 4112 cm^-1 for H ₂/Na-ZSM-5 and H₂/K-ZSM-5, respectively. Thermodynamic characterization of the adsorbed state was carried out by means of variable-temperature infrared spectroscopy; simultaneously measuring integrated band intensity, temperature and equilibrium pressure of the gas phase. For the H₂/Na-ZSM-5 system, the standard adsorption enthalpy and entropy resulted to be ΔH° = -10.3 (±0.5) kJ mol^-1 and ΔS° = -121 (±10) J mol^-1 K^-1. For H ₂/K-ZSM-5 corresponding values were -9.1 (±0.5) kJ mol ^-1 and -124 (±10) J mol^-1 K^-1, respectively.