The parameters governing solid-phase microextraction (SPME) are investigated, with emphasis on the determination of the partition coefficients, K, and their use to predict the selection of a fibre, depending on the respective characteristics of the fibres and the analytes. Film thickness and stability of the compounds can interfere with the determination of K values. The time profile curves were determined for twelve pesticides having a wide range of water solubilities and polarities and using four fibres (polydimethylsiloxane, polydimethylsiloxane-divinylbenzene, Carbowax-divinylbenzene and polyacrylate). Although the affinity order was different for the four coatings, no correlation was found between the respective characteristics of the fibres and solutes. The two fibres containing divinylbenzene were shown to have the highest affinities and the polydimethylsiloxane had the lowest affinity. The polyacrylate fibre which is the more polar commercial fibre did not provide the highest affinities for the more polar and water-soluble analytes. The important parameters for quantitative analysis have been evaluated. The calibration curves were similar when one analyte of interest was present on its own in a drinking water sample, or when eleven other pesticides were present at the same concentration or when much higher concentrations of other analytes were present in the sample. Linearity was obtained over a wide range of concentrations in drinking water samples. Detection limits are in agreement with European regulatory levels in drinking water for most of the analytes using solid-phase microextraction-gas chromatography-nitrogen-phosphorus detection (SPME-GC-NPD). In contaminated surface water samples, the chromatograms are relatively clean and most of the compounds can be detected at levels lower than 0.5 microgram/l.