Liquid-phase concentrations of volatile compounds were determined on-line via gasphase analysis using mass spectrometry. It has been shown theoretically as well as experimentally that almost every volatile compound commonly produced by microorganisms can be measured by the method presented, if its partial pressure in the fermenter off-gas exceeds 1 mu bar. This has been shown in a baker’s yeast as well as a Bacillus subtilis culture. The experimental results also indicated that these measurements are not controlled by mass-transfer kinetics. A dimensionless parameter, E, allows us to check whether the analysis is determined by thermodynamics (Henry coefficient) or by mass-transfer rates.