[Al]- and [Ga]-HZSM-5 having closely similar acid site densities were prepared. Temperature-programmed desorption of ammonia (TPDA) proved the known acid strength sequence : [Al] > [Ga]. Frequency shifts of the OH stretching mode to lower frequencies were induced upon CO adsorption at 77 K due to OH...CO H-bonding interactions. These frequency shifts are a measure of the H-bond donor strength of the OH groups and hence, of their acidic strength. The observed shifts of 313 and 282 cm(-1) for the [Al]- and [Ga]-silicate, respectively, clearly reflect their relative acid strength. [Ga]-HZSM-5 develops an acid strength similar to that found for HNaY at high exchange levels. Evidence is presented for an intrinsic heterogeneity of acid properties of the acidic hydroxyls probably caused by inhomogeneous distributions of the trivalent cations in the framework.