We apply, for first time, the recently developed proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) apparatus as a rapid method for the monitoring of lactic acid fermentation (LAF) of milk. PTR-TOF-MS has been proposed as a very fast, highly sensitive and versatile technique but there have been no reports of its application to dynamic biochemical processes with relevance to the food industry. LAF is a biochemical-physicochemical dynamic process particularly relevant for the dairy industry as it is an important step in the production of many dairy products. Further, LAF is important in the utilization of the by-products of the cheese industry, such as whey wastewaters. We show that PTR-TOF-MS is a powerful method for the monitoring of major volatile organic chemicals (VOCs) formed or depleted during LAF, including acetaldehyde, diacetyl, acetoin and 2-propanone, and it also provides information about the evolution of minor VOCs such as acetic acid, 2,3-pentanedione, ethanol, and off-flavor related VOCs such as dimethyl sulfide and furfural. This can be very important considering that the conventional measurement of pH decrease during LAF is often ineffective due to the reduced response of pH electrodes resulting from the formation of protein sediments. Solid-phase microextraction gas chromatography/mass spectrometry (SPME-GC/MS) data on the inoculated milk base and final fermented product are also presented to supporting peak identification. We demonstrate that PTR-TOF-MS can be used as a rapid, efficient and non-invasive method for the monitoring of LAF from headspace, supplying important data about the quality of the final product and that it may be used to monitor the efficacy of manufacturing practices.