Abstract: The disclosure relates to a process for the aerobic, thermophilic degradation, in the liquid phase, of microbially degradable substances. In the process, the particle size of the substance is reduced before the treatment to be at most 50 mm, and the substance is mixed with a liquid to a dry content of at most 15% by weight. The pH of the resultant slurry is adjusted to at most 12. Enzyme is added to promote degradation prior to preheating. The slurry is then pre-heated and thereafter subjected to a continuous, aerobic, substantially thermophilic degradation treatment in the presence of air for at most 10 days. The temperature of the slurry is regulated to from 40 to 80.degree. C. Finally, active sludge from the treatment is utilized for inoculating untreated slurry and ready-treated slurry is dewatered and possibly enriched with nutrient.