Hydrolysis has been proved as a key factor on biological treatment process like anaerobic digestion or water purification facilities. Regarding to composting, some previous works made with vegetable fibres has demonstrated the slow speed of this process for this pool of organic matter (Tuomela et al, 2000), neverless, the experiences related to this process are only a few (Komilis, 2006). The present work deals with the study of the hydrolysis process into the composting by the following of the degradation of several organic matter pools at four key temperatures of the composting process (20, 37, 50 and 60ºC). The waste used in this work was a mixture which contained all the main groups in which the organic matter can be divided, this means several kinds of carbohydrates, proteins and lipids; this was the reason a sewage sludge from a fish processing factory was chosen, with poplar bark as bulking agent. The mixture was introduced into independent experimental containers (N=15), which were maintained at the different temperature treatments during 15 days. A destructive sampling was made at the 1, 3, 6, 10 and 15 days of the process. This experimental design showed to be rather adequate to the process study, being obtained useful data which shows that the degradation of each component of the organic matter took place at different process moments, and not as a homogeneous whole. Composting, organic matter degradation dynamics, fishery sludge.