A dynamic model for the composting process has been developed, which integrates several biochemical and physical processes. Different microbial populations (mesophilic and thermophilic bacteria, actinomycetes and fungi) have been considered, each specialized in certain types of polymeric substrates (carbohydrates, proteins, lipids, hemicellulose, cellulose and lignin) and their hydrolysis products. Heat and mass transfer between the three phases of the system have been taken into account. The gas phase was considered to be composed by nitrogen, oxygen, carbon dioxide, ammonia and water vapour. Model computer simulations provided results that fitted satisfactory the experimental data. A sensitivity analysis was performed to determine the key parameters of the model. The partition of both the composting mass and the active biomass into different major groups of substrates and specialized microbial populations, as well as the factors affecting the gas-liquid equilibrium, were important for an accurate description of the composting process.