Thermogravimetry/mass spectrometry (TG/MS) was applied to eight different sewage sludge samples digested anaerobically and aerobically. The samples were thermally degraded in a nitro en atmosphere using a heating rate of 20 degrees C/min. The mass loss and the evolution profiles of the thermal decomposition products of molecular mass less than 100 Da were monitored. The thermal loss occurs in three stages, centered around 250 degrees C, 350 degrees C and 550 degrees C, producing high quantities of gases, as hydrogen, water, hydrocarbons (C-1-C-4, both saturated and unsaturated), methanol, chloromethane, carbon dioxide and acetic acid. The production of acetic acid during aerobic treatment is much higher than during anaerobic treatment. On the other hand, the organic matter that is biodegraded in the aerobic process produces a series of compounds that, when pyrolyzing, does not form char.