After investigating the application of the mesophilic and thermophilic processes in completely stirred, batch, and plug-flow reactors, in this study the authors consider the anaerobic fermentation of source-sorted organic municipal solid wastes in psychrophilic conditions (1422 ° C) without pH control. The pilot-scale reactor was operated in a batch mode, with a hydraulic retention time of 4-4.5 d. The production of soluble COD from the particulate matter was (on average) 0.27 gCOD per gram of total volatile solids fed to the reactor when operating with a total solids content of 20-35 g/L. The volatile fatty acids (VFA) were 15% of the soluble COD produced after 4 d of reaction. These values are far lower than those found in mesophilic and thermophilic conditions, where the production of soluble COD ranged from 0.5 up to 0.9 gCOD/ gTVS(fed) and volatile fatty acids could reach 90% of soluble COD. Further, the first-order reaction constant for the hydrolysis process, K-h, for the psychrophilic conditions was found equal to 0.11 d(-1) at 20 ° C, while it was in the range 0.2-0.4 d(-1) when operating in mesophilic or thermophilic conditions. Conclusively, the study of the psychrophilic fermentation process allowed for completing the scenario of different options of anaerobic solid-state fermentation of organic waste. Though mesophilic and thermophilic processes resulted in being more effective in dissolution of particulate matter, psychrophilic processes can be of some interest because they are simpler and energy saving. In particular, psychrophilic processes can be useful for the production of rough soluble COD to be used, e.g., for sustaining the biological nutrients removal processes in wastewater treatment.