The pyrolysis and combustion of 17 sewage sludges were studied by thermogravimetry. The dynamic experiments were carried out at 10 8C min À1 , and three different behaviours were observed at the decomposition curves. One of them shows combustion and pyrolysis curves that are parallel until high temperatures, and so the combustion process can be considered as an oxidative pyrolysis. The DSC combustion curve shows that exothermic processes take place during most of the combustion run. The second behaviour shows combustion and pyrolysis curves that approximately coincide at low temperatures, but at high temperatures the combustion curve falls strongly, indicating that the combustion really occurs at high temperatures. The DSC curve shows that the exothermic process corresponds only to the combustion process. Some sludges with this behaviour show a combustion curve above the pyrolysis curve at a range of temperatures that also happens to BSA protein. The third behaviour is intermediate between the first and second behaviours, where the DSC curve shows exothermic processes during the first part and during the combustion of the char formed. Total nitrogen content, nitrogen content in decomposed fraction from combustion and a parameter that is associated to the decomposition of proteins were also studied and it was observed that there is a relationship between all of them and the type of behaviour in thermogravimetry.