Imidazolium-based ionic liquids with [PF6](-), [BF4](-) and [GaCl4](-) counterions have been synthesised and used in hydroethoxycarborylation of styrene. In addition to the formation of the expected ethyl 2-phenyl- and 3-phenyl-propionates, the oligomerization (polymerization) of the substrate depending on the counterion of the ionic liquid was also observed. Moderate to high regioselectivities towards branched esters have been obtained with the 'preformed' PdCl2 (PPh3)(2) catalyst. The addition of 1, l'-bis(diphenylphosphino)ferrocene (dppf) favoured the formation of the linear isomer, however, complete linear regioselectivity has been obtained only in acetonyl-methyl-imidazolium hexafluorophosphate. It was found that the regioselectivity is strongly influenced by the structure of the ionic liquid: in [BMIM] [PF6] the formation of the linear ester is slightly favoured, however, the substitution of the butyl substituent with benzyl or acetonyl group resulted in complete branched selectivity. (c) 2005 Elsevier B.V. All rights reserved.