Ammonium and cesium derivatives from H3PW12O40 (H3PW), namely (NH4)xCs(2.5-x)H(0.5)PW(12)O(40) (x = 0.5, 1, 1.5, 2), were synthesized and structurally characterized by FT-Raman spectroscopy, and their thermal stability was evaluated by FTIR and TGA/DTA. The acidity was characterized by the adsorption/desorption of gaseous pyridine, by FTIR and TGA/DTA as well as by the reaction of oleic acid and ethanol. The stability of the mixed salts regarding the Keggin structure was much higher than the parent acid, with the onset decomposition around 520 degrees C. Nonetheless, calcination up to 300 degrees C is recommended for the integrity of the mixed salt. The FTIR of adsorbed pyridine displayed only Bronsted acidic sites, which was confirmed by TGA measurements of the formation of Py-H+. . .Py adducts. The best esterification result was for (NH4)(2)Cs0.5H0.5PW12O40 with TOF = 1.314 mol(EO) mol(-1) proton s(-1) with a 1:6 (oleic acid:ethanol) molar ratio, at 80 degrees C and 10 wt% catalyst in relation to the acid. (C) 2012 Elsevier B.V. All rights reserved.