A direct insertion membrane probe for the direct determination of organic compounds in aqueous samples has been designed and built. This device allows the transportation of analyte solution over a sheet membrane via which pervaporation transports compounds directly into the heated ion source of a mass spectrometer. This configuration, together with the possibility of temperature control at the solution/membrane interface, decreases response times (typically <10 s) and memory effects and gives very good sensitivity and low detection limits (down to 10 μg l−1). Optimization of different parameters which affect the performance of the device is reported. High probe temperatures and high flow-rates are generally preferred, giving increased sensitivity. A knowledge of the sample matrix is essential in precise quantitative analysis, because matrix components can change permeation rates and ionization efficiencies. The usefulness of the device in bioreactor monitoring and in environmental analysis is demonstrated.