The acidic properties of CF3SO3H/SiO2, acid catalysts have been investigated by the protonation of weak bases (B) (B+H+ reversible arrow BH+) and the proton-transfer process from (H(+)A(-)) to (BH(+)A(-)) has been analysed by a thermodynamic procedure used to account the variation of the activity coefficient terms of the species involved. Acid-base systems with different substituted pyridines as back-titrating agents of BH+ (i.e. BH+ --> B) have also been studied and the changes observed in the acid-base interactions according, to basicity of pyridines are discussed. The results in solid phase have been compared with those observed in concentrated aqueous solutions (i.e. CF3SO3H + H2O) where "acidity" and "protonating ability" have been distinguished as parameters of interest in the description of nonideal acid systems. Silica loaded with CF3SO3H, with H2SO4 and with a mixture of both acids have been tested in acid catalysed reactions and their catalytic effectiveness has been explored towards substrates with high acid requirements for the conversion reagents-products. From the available observations in liquid and in solid phase, CF3SO3H has been proved to be a less effective acid catalyst than expected from the observed protonating ability of acidic medium. Strong interactions between ionic species and the involvement of ion-pairs in concentrated acid systems have been suggested.