For the optimization of fed-batch fermentations of methylotrophic organisms an online measuring method is presented, with which the methanol concentration in the medium during a fermentation can be determined by a purge gas pervaporation principle. In contrast to other analytical methods, this measuring method offers the possibility of maintaining the substrate concentration at a controlled value in processes with methanol as the central substrate. Difficulties, but also their overcoming in the adaptation of the measuring method to fermentation processes are described.