The metal carbonyls [M(CO) ₆ ] (M = Mo or W) were found to be molecularly adsorbed on the surface of H-ZSM-5 zeolite crystals. Three types of adsorbed species were detected by IR spectroscopy: weakly physisorbed metal carbonyls and [M(CO) ₆ ] molecules interacting (via a CO ligand) with OH groups or with Lewis-acid sites (co-ordinatively unsaturated Al ³⁺ ions). For physisorbed species a single C–O stretching mode (T _1u ) was observed, around 1990 cm ^-1 . The (OC) ₅ MCO···HOSi species gave a band at 1965 cm ^-1 (anchored CO ligand) and the corresponding E + 2A ₁ modes at higher frequency. O-Bonding between the metal hexacarbonyls and Lewis-acid centres gives rise to a characteristic IR absorption band in the range 1750–1850 cm ^-1 , and corresponding E + 2A ₁ modes at _CO > 2000 cm ^-1 . Close inspection of these spectroscopic features, and comparison with IR spectra of adsorbed CO, enables characterization of the zeolite surface. For medium-pore zeolites an important feature of this method is that [M(CO) ₆ ] molecules cannot penetrate inside the zeolite channels, therefore discrimination between internal and external surface sites can be accomplished. Thus, it was found that both Brønsted-acid sites [bridged Si(OH)Al groups] and silanols are located mainly inside the zeolite channels.