The surface acidic properties of various solid catalysts (i.e., clay montmorillonite, acid modified smectite clay, HY zeolite, sulphate-promoted ZrO2 (SO42-/ZrO2) and Cs-exchanged dodecatungstophosphoric acid (H0.5Cs2.5PW12O40)) have been comparatively evaluated by temperature programmed desorption (TPD) measurements of adsorbed NH3, C5H5N and C6H6. Mathematical analysis of TPD spectra of adsorbed ammonia indicates the presence of weak, medium and strong acid sites in all the catalysts. On the basis of the TPD patterns of NH3, C5H5N and C6H6, the nature (Bronsted and Lewis) of the acid sites has been ascertained. Catalytic measurements in CH3OH dehydration and benzene alkylation indicate that besides the acidic properties, the diffusional phenomena also play a key role on the reactivity of solid acid catalysts. The superior catalytic performance of the H0.5Cs2.5PW12O40 system in the above reactions has been explained in the light of a pseudo-liquid behaviour.