The aldol condensation is taken as a model reaction in order to study both the acid site density and the pore dimension on a variety of acidic molecular sieves (small-, medium-, large-pore zeolites and amorphous micro-, mesoporous silica-aluminas). Adsorption-reaction experiments of acetone are followed by W-Vis spectroscopy. The main evidences obtained in this work allow us to relate: (i) the shift of the UV signal of acetone in the earliest stage of reaction to the strength of the Bronsted acid sites of the molecular sieves; (ii) the acetone conversion at 21 degrees C to the density of the Bronsted acid sites; (iii) the product distribution to the dimensions of the pores of the molecular sieves. Medium pore zeolites give phorone as final product, while zeolites with large channels and mesoporous silica-aluminas favor the formation of isophorone.