The acidic properties of two samples of zeolite Theta-1 (Si/Al = 25 and 50) have been studied as the propensity to form hydrogen bond with several probes. The adsorption of small molecules (N-2 and CO) at a nominal temperature of 77 K leads to bathochromic shifts in the OH stretching vibration of the Si(OH)Al groups close to those observed with ZSM-5 and H-MOR, so suggesting comparable acidity. The corresponding values measured with propene, a bulkier molecule, are instead somewhat smaller than those observed with the same reference zeolites. This is ascribed to the proximity of the walls surrounding the Si(OH)Al site, with which secondary (van der Waals) interactions may occur, hampering the purely hydrogen-bonding phenomenon. This same geometric constraint may be the reason for the unexpected absence of proton transfer to propene and for the exceptionally low OH stretching, frequency (3598 cm(-1)) of free Si(OH)Al species. No Lewis acidity was found. With the sample richer in Al some dealumination took place; because of their mono-dimensional nature, a remarkable fraction of channels are blocked by a relatively small amount of debris, with the exclusion of a sizable fraction of Si(OH)Al sites.