A series of insoluble heteropolytungstate (H3PW12O40 HPW) salts, CsxH3-xPW12O40 (x = 0.9-3), were synthesized and characterized using a range of bulk and surface sensitive probes including N-2 porosimetry, powder XRD, FTIR, XpS, P-31 MAS NMR, and NH3 calorimetry. Materials with Cs content in the range x = 2.0-2.7 were composed of dispersed crystallites with surface areas similar to 100 m(2) g(-1) and high Bronsted acid strengths [Delta H-ads(0)(NH3) = -150 kJ mol(-1)], similar to the parent heteropolyacid. The number of accessible surface acid sites probed by alpha-pinene isomerization correlated well with those determined by NH3 adsorption calorimetry and surface area measurements. CsxH3-xPW12O40 were active toward the esterification of palmitic acid and transesterification of tributyrin, important steps in fatty acid and ester processing for biodiesel synthesis. Optimum performance occurs for Cs loadings of x = 2.0-2.3, correlating with the accessible surface acid site density. These catalysts were recoverable with no leaching of soluble HPW. (c) 2007 Elsevier Inc. All rights reserved.