The effect of Bronsted acid site strength on the liquid-phase conversion of pseudoionone to ionone isomers (alpha-, beta- and gamma-ionone) was studied on resin Amberlyst 35W, silica-supported heteropolyacid (HPAS) and silica-supported triflic acid (TFAS). Catalyst acidity was probed by temperature-programmed desorption of NH(3) coupled with infrared spectra of adsorbed pyridine. The initial pseudoionone conversion rate followed the order: TFAS > Amberlyst 35W approximate to HPAS. Synthesis of the three ionone isomers occurred via a common cyclic carbocation intermediate formed from the activation of the pseudoionone molecule on Bronsted acid sites. Initial ionone mixtures containing a alpha:beta:gamma isomer distribution of about 40:20:40 were formed, irrespective of the acid site strength. But the ionone mixture composition changed with the progress of the reaction because gamma-ionone was consecutively converted to a-ionone on HPAS and Amberlyst 35W, whereas the stronger acid sites of TFAS converted gamma-ionone to beta-ionone. (C) 2009 Elsevier B.V. All rights reserved.