The dielectric properties of water solutions of ethanol and sugar are investigated in the microwave region with the objective of setting up a method for the quality control of the fermentation process of alcoholic beverages. Alcoholic fermentation is the process by which carbohydrates, in particular sugar, are converted by the yeast into alcohol. During that process several other by-product compounds are produced, including a significant amount of carbon dioxide. The fermentation stage is of fundamental importance in the production of alcoholic beverages because some of the by-products' components have a considerable effect on the flavour, aroma, and other characteristic properties of the beverages. The on-line monitoring of the fermentation process can thus be very useful for controlling the timing and the development of the process in order to correct it earlier if deviations from "normality" occur. Dielectric spectroscopy is shown to be suitable for such a task, being able to discriminate between the initial water-sugar mixture and the final water-alcohol solution and making it possible to detect the production of carbon dioxide during fermentation. A case-study consisting of the monitoring of the primary fermentqtion of beer by dielectric spectroscopy is presented and discussed.