The performance of a membrane introduction mass spectrometry (MIMS) system has been tested under pilot plant conditions. The work employed a Finnigan ITS-40 ion trap instrument, adapted to MIMS experiments by the addition of an external membrane/jet separator interface and packaged to operate in an industrial production environment. A representative sample from a 9000 L fermentation reactor was taken continuously and monitored on-line, using a stainless steel tangential filter as the only sample pretreatment. This filter was capable of withstanding the temperature and pressure of 30 psig steam sterilization as well as the 10-30 psig operating pressure from a sample stream containing in excess of 10% solids. The filtrate was sampled using a flow injection analysis system which allowed quantification using external standards. Calibration experiments established that the system displayed a linear response to ethanol at concentrations between 1 and 10% by volume. During subsequent on-line experiments, ethanol standard and sample streams were examined alternately. The response of the standard solutions was used to quantitate the response of the sample stream and reduce errors associated with long term instrumental drift. Ethanol concentrations were found to be 2.97 Β± 0.07% and were quantitatively in agreement with off-line high-performance liquid chromatographic data. Minor components identified during on-line reactor monitoring included acetic and lactic acid. After one week of off-line operation in the pilot plant, on-line monitoring was continued for a period of four days.