In Catalunya every year more than three hundred thousand tons of sewage sludge are produced, making its management a priority. One of the ways in which these wastes are given a way out is through their agricultural recovery. In order to give them an agronomic exit, these wastes must meet the requirements set by the Agència de Residus de Catalunya. Within the requirements set by the regulatory body, a minimum stability is required by the sludge. Several methods exist to measure stability, but l'Agència Catalana de l'Aigua uses the Klasson method (MAB 1971) to evaluate the sludge generated in Catalunya. The objective of this paper is to compare the degrees of stability obtained by analyzing nine samples of sludge and one of compost using the Klasson method (MAB 1971) and the van Soest method (Binner 1997). With the results obtained, we observe that the stability obtained by the van Soest method (Binner 1997) is significantly lower than that obtained with the Klasson method (MAB 1971). A fact that could be explained by the nitrogen content of the sludge, since Norman and Jenkyns (1934) observed that when samples with a high nitrogen content were treated with the Klasson method, a precipitate was produced that leads to an overestimation of the recalcitrant organic matter content of the sample. That said, we believe that more studies are needed to verify the reliability of the Klasson method (MAB 1971) to determine the stability of organic matter in sewage sludge