The importance of Bronsted acid sites for partial oxidation of propane to acetone in CaY was investigated by in Situ FTIR spectroscopy. With an increasing number of protons in Ca-Y, Volcano plots were observed for (1) amount of adsorbed propane; (2) initial acetone formation rate; (3) total amount of acetone after 20 h of reaction; (4) acetone selectivity. The results clearly show that Bronsted acidity increases the isopropylhydroperoxide (IHP) formation rate, most likely via a catalytic H+ transfer, and IHP decomposition into acetone and water. Moreover, with increasing IHP concentration 2-propanol was found in addition to acetone. Since an increasing number of Bronsted acid sites ratio was observed for both activity and selectivity for acetone. implies a decreasing number of Ca2+ in the zeolite, an optimal H+/Ca2+ (c) 2005 Elsevier Inc. All rights reserved.