Membrane inlet mass spectrometry has been applied for continuous on-line waste water monitoring at a waste water purification plant of an oil refinery. The instrumentation includes a fully automatic sample delivery system and a membrane inlet mass spectrometer. The entire system is controlled with a custom-made automation program from a remotely located personal computer. Membrane inlet mass spectrometry was selected for this task since it was necessary to be able to do qualitative and quantitative analysis of the volatile organic contaminants at low concentration levels. The developed measurement system has allowed long-term continuous collection of qualitative and quantitative data about the contaminants, typically BTEX compounds, of the waste water stream.