The microbiological aspects of a novel process of grape marc composting have been investigated. It has been possible to determine the succession of populations during the process which are required to obtain the required final product. The initial population comprises exclusively yeasts which, by autolysis and subsequent binding of the residual alcohol by esterifying reactions, enable rapid appearance of a mixed population of bacteria. The temperature increase continued by this bacterial flora favours growth of a thermophilic fungal flora, which is mainly responsible for the microbial decomposition process. The most important organism is Thermomyces lanuginosus Tsiklinski. Final humification is effected by a mixed population of Streptomycetes. It was possible to optimize the process by installing heat exchangers, thereby creating optimum conditions for the most important organism, T. lanuginosus.