Cs exchanged silicotungstic acid catalysts of general formula CsxH4-xSiW12O40 (x = 0.8-4) have been synthesised and characterised by a range of techniques including elemental analysis, N-2 gas adsorption, XRD, XPS and NH3 flow calorimetry. Cs substitution promotes recrystallisation of the parent H4SiW12O40 polyoxometallate to the Cs-4 salt, via a stable intermediate phase formed at compositions between Cs0.8-2.8. This recrystallisation is accompanied by a pronounced rise and subsequent fall in porosity, with a maximum mesopore volume obtained for materials containing 2.8 Cs atoms per Keggin unit. Calorimetry reveals all CsxH4-xSiW12O40 are strong acids, with Delta H-ads(theta)(NH3) ranging from -142 to 116 kJ mol(-1) with increasing Cs content, consistently weaker than their phosphotungstic analogues. CsxH4-xSiW12O40 materials are active catalysts for both C-4 and C-8 triglyceride transesterification, and palmitic acid esterification with methanol, For loadings <= 0.8 Cs per Keggin, (trans)esterification activity arises from homogeneous contributions. However, higher degrees of substitution result in entirely heterogeneous catalysis, with rates proportional to the density of accessible acid sites present within mesopores. (C) 2009 Elsevier B.V. All rights reserved.