Composting is a biological process of decomposition of organic materials in an aerobic environment, which modifies the chemical composition and the thermal behavior of biomass. During composting, fungi and bacteria promote the decomposition of hemicellulosic and cellulosic fractions, increasing the lignin proportion. Its product, compost, is usually used as an amendment to soil; however, its physicochemical characteristics turn it into an interesting feedstock in pyrolysis or gasification facilities. The changes that composting produces on biomass pyrolysis can be explained using an autocatalytic kinetic model (Prout-Tompkins). Thus, by means of a similar set of kinetic parameters for both the biomass and compost, it is possible to simulate the thermogravimetric analysis data (TG and DTG curves) of the materials as a sum of thermal degradations of its main pseudocomponents, hemicellulose, cellulose, lignin, and extractives. TG analysis coupled to mass spectrometry (MS) allows monitoring of the gas production during pyrolysis. Water and carbon oxide MS profiles can be simulated by an optimized linear combination of previously calculated DTG curves of pseudocomponents; however, in order to simulate the hydrogen MS signal, it is necessary to consider the char obtained in the course of the volatilization process. During pyrolysis, hydrogen production has two origins, volatilization of biomass pseudocomponents and charring. The last mechanism explains approximately 75% of the hydrogen obtained from compost. The pseudocomponent that produces more hydrogen by weight unit is lignin, showing a specific hydrogen production much higher than carbohydrates (3:1:8 for hemicellulose/cellulose/lignin). This fact, together with the greater lignin content in compost, explains the positive effect of composting on hydrogen production.