A procedure for the determination of seven parabens (esters of 4-hydroxybenzoic acid), including the distinction between branched and linear isomers of propyl- and butyl-parabens and triclosan in water samples, was developed and evaluated. The procedure includes in-sample acetylation-non-porous membrane-assisted liquid-liquid extraction and large volume injection-gas chromatography-ion trap-tandem mass spectrometry. Different derivatisation strategies were considered, i.e. post-extraction silylation with N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide and in situ acylation with acetic anhydride (Ac(2)O) and isobutylchloroformate. Moreover, acceptor solvent and the basic catalyser of the acylation reaction were investigated. Thus, in situ derivatisation with Ac(2)O and potassium hydrogenphosphate (as basic catalyser) was selected. Potassium hydrogenphosphate overcomes some drawbacks of other basic catalysers, e.g. toxicity and bubble formation, while leads to higher responses. Subsequently, other experimental variables affecting derivatisation-extraction yield such as pre-stirring time, salt addition and volume of Ac(2)O were optimised by an experimental design approach. Under optimised conditions, the proposed method achieved detection limits from 0.1 to 1.4 ng L(-1) for a sample volume of 18 mL and extraction efficiencies, estimated by comparison with liquid-liquid extraction, between 46% (for methyl- and ethyl-parabens) and 110% (for benzylparaben). The reported sample preparation approach is free of matrix effects for parabens but affected for triclosan with a reduction of approximately 40% when wastewater samples are analysed; therefore, both internal and external calibration can be used as quantification techniques for parabens, but internal standard calibration is mandatory for triclosan. The application of the method to real samples revealed the presence of these compounds in raw wastewater at concentrations up to 26 ng mL(-1), the prevalence of the linear isomer of propylparaben (n-PrP), and the coexistence of the two isomers of butylparaben (i-BuP and n-BuP) at similar levels.