A sensor-type, multi-compound monitoring system was constructed to measure several substances dissolved in a culture medium by use of a membrane inlet system coupled with a low-cost quadrupole mass spectrometer. The membrane inlet system was shown to have higher sensitivity with an even faster response than a capillary inlet system. The system was able to monitor on-line gaseous (H-2, CO2 and O-2) and volatile (ethanol, butanol, acetone, acetic acid and ethyl acetate) compounds at concentrations in a range suitable for fermentation process monitoring applications. The effects of temperature, agitation speed and pressure on measurements using the membrane inlet system were investigated. For volatile compound measurements, temperature and pressure had an effect on the response but the agitation speed did not. A high concentration of compounds caused measurement saturation due to the space charge effect, but this could be overcome by reducing the surface area of the membrane. The system was applied to the monitoring of ethanol in a yeast cultivation. Measurements were affected by CO2 produced during the fermentation, but this could be compensated for by means of a linear empirical equation. The system demonstrated high potentiality for application to the on-line monitoring of multiple compounds in fermentation processes.