The appearance of catalyst-reactant interactions observed in on-line ATR FT-IR spectra of Nafion/silica catalysed esterification and etherification reactions of 1-octanol was investigated. It was assessed by variation of catalyst and solvent that the catalyst-reactant band is a result of the reaction of silica with 1-octanol, yields Si-O-R functionalities. Based on off-line TPD-MS and TGA characterisation of the used solid catalyst powder, the formation of Si-O-R linkages on the SiO2 surface was confirmed. This demonstrates conclusively that in specific liquid-phase reactions, on-line analysis of intermediate species adsorbed on heterogeneous catalysts is possible. Using on-line particle analyzers, it was assessed that the absorptions were not the result of severe attrition of the catalyst particles in the course of the reaction, but apparently the result of apolar interactions of the particles with the diamond crystal.