[Al]-, [Ga]- and [Fe]-HZSM-5 having closely similar Bronsted acid site densities were prepared. The low-temperature adsorption of H-2 and D-2 was studied by FTIR spectroscopy and the frequency shifts Delta upsilon(OH) and Delta upsilon(HH) (Delta upsilon(DD)) were measured and compared with corresponding frequency shifts observed for CO and N-2 probes. A linear correlation between Delta upsilon(OH)(1/2) which is proportional to the heat of formation Delta H-B of the H-bonded complex O-H...B (B representing the H-bond acceptor), and the proton affinities of the three probe molecules was found for [Al]-HZSM-5. The Delta upsilon(OH)-values measured for CO and H-2 (D-2) On the three isomorphously substituted zeolites suggested the following acid strength ranking : [Al]-HZSM-5 > [Ga]-HZSM-5 approximate to [Fe]-HZSM-5. This sequence is clearly reflected in the relative activities of the materials for the acid-catalyzed disproportionation of ethylbenzene.