A continuous on-line analysis system for components of the medium during the anaerobic and aerobic fermentation of glucose by Saccharomyces cerevisiae is described. The procedures used include: analysis of the outlet gas by conventional analyzers (CO2, O2) and a mass spectrometer; continuous sampling of the liquid phase with microfiltration and subsequent determination of phosphate and glucose (spectrophotometer), glucose (polarimeter) and ammonium ion (ion-sensitive sensor); and continuous separation of volatile components and dissolved gases with diffusion through a silicone rubber membrane in high vacuum and determination of mass (m/z=18, 28, 31, 32, 40, 44) by means of a mass spectrometer. The system was tested on a batch reactor and a continuously operated three-stage cascade consisting of stirred tank reactors. This equipment was connected to a process computer. Glucose was quantified either polarimetrically or spectrophotometrically with p-hydroxybenzoic acid hydrazide; the latter was far more sensitive and was preferred to enzymatic methods, with which correlation was good. Only acetoin was found to interfere. Problems with operating a mass spectrometer for fermentation control in the exit stream and with membrane sampling are discussed.