A sampling system for on-line monitoring of organic compounds of low volatility in complex fermentation media uses membrane inlet mass spectrometry (MIMS). A syringe pump draws a continuous flow of microfiltered broth from the reactor and circulates it after acidification through a membrane inlet, in which a membrane is the only interface between the sample and the high vacuum of a mass spectrometer. All operations run automatically, i.e., sampling, acidification, measurement, and calibration. The on-stream acidification enables MIMS monitori ng of carboxylic acids, as they must be undissociated in order to pass the hydrophobic membrane. The performance of the monitoring system was tested by measurements of standard solutions of phenoxyacetic acid (POAA, the side chain precursor of penicillin-V) as well as on POAA during 200 h penicillin-V fermentation. During the entire fermentation POAA was monitored in low millimolar concentrations with high accuracy and fast response to step changes in POAA concentration. Tandem mass spectrometry (MS/MS) allowed direct identification of peaks in the mass spectrum of the broth that were not accounted for by POAA. These peaks were identified as SO2 and SCO.