The results of a systematic study on the precision and repeatability of measurements of the headspace solid phase micro-extraction (SPME) with open-cap vials in combination with capillary gas chromatography in comparison with septum-sealed vials are reported. Benzene, toluene, ethylbenzene, and xylene isomers (BTEX) were used as the target analytes in the investigation of spiked water samples at concentration levels of 42.5 microg l(-1). The dependence of a sample volume versus peak area showed maximum SPME recovery. The influence of sample volume on the precision and the time of taking the sample on the losses of volatile analytes was examined.