Esterification of phthalic, anhydride with 2-ethylbexanol and 1-butanol. and ester decomposition of dioctyl phthalate (DOP) in presence of Keggin; H3PW12O40, H4SiW12O40, H4SiMo12O40, Wells-Dawson; H6P2W18O62, H6P2W17MoO62 and Preyssler; H-14[NaP5W29MoO110], H-14[NaP5W30O110], type heteropolyacids have been investigated. The heteropolyacids with Preyssler and Wells-Dawson structures and their molybdenum substituted derivatives show higher activity in esterification and ester decomposition reactions than Keggin type heteropolyacids. A complete conversion of phthalic anhydride to dioctyl phthalate and dibutyl phthalate are achieved in 2 h in presence of molybdenum substituted Preyssler heteropolyacid. In the decomposition of dioctyl phthalate in the presence of Preyssler heteropolyacid, 2-ethylhexene is formed in quantitative yield. (C) 2003 Elsevier Science B.V. All rights reserved.