The acidic and sorption properties of Al containing MCM-41 molecular sieves in comparison with relevant FAU and MFI zeolites were investigated by n-hexane, benzene, acetonitrile, and water adsorption by means of microcalorimetry, isotherm measurements as well as FTIR and H-1 MAS NMR techniques, using acetonitrile or chloro-acetonitrile as probes. MCM-41 shows relatively weak Bronsted acidity, comparable with the LF proton of HY. The strongest sites are Lewis centres according to the strongest heats of adsorption and the corresponding characteristic CN frequencies of acetonitrile in contrast to the HY or the dealuminated form of HY. MCM-41 shows a physisorption behaviour of a well-defined mesoporous molecular sieve, which makes it favourable as a model substance. Whereas, the heats of adsorption of acetonitrile are mainly influenced by a specific interaction on the acidic sites in the meso- and microporous molecular sieves, the adsorption heat of a non-polar molecule like n-hexane is determined by the pore size or density of those materials. However, a pore-size effect on the adsorption heats of acetonitrile in acidic molecular sieves has to be taken into account while employing those heats as a measure of acidic strength. The contribution of the pore size governed dispersion interaction in MCM-41 is ca. 15 kJ/mol smaller than that in the narrow channels of MFI.