The design and features of an on-line purge and cold-trap preconcentration device for rapid analysis of volatile organic compounds in aqueous samples are discussed. Excessive water is removed from the purge gas by a condenser or a water permeable membrane in order to avoid blocking of the capillary cold-trap. Synthetic mixtures covering concentrations ranging from tenths to tens of ppb's and different chemical classes are used to study the effect of various process factors on the efficiency and selectivity of water removal as well as on the purging recovery. The importance of the concentration of the solutes, the flow rateinconjunction with thevolumeof the purge gas, and the temperatureof thecondenser, thecold-trapand the sample is emphasized. Theoretical models describing the purge process and the blocking of the cold-trap agree fairly well with the highlyreproducibleexperimental results (o = 2-4'10). Both the condenserand the Nafion membrane successfullyremove water, although some compounds, dependent on volatility and polarity, are partly or completely lost. It is shown that non-polar volatile organic compounds are efficiently enriched so that recoveries between 80-100% and a detection limit of 1 ppt can be obtained. The applicability of the system is illustrated on some examples.