Pyrolysis of dried sewage sludge samples treated with the additives silica zeolite, calcium oxide, dolomite, ammonium sulphate or diammonium sulphate were conducted by thermal gravimetric anaysis (TGA). The pyrolysis of the untreated sewage sludge showed four regions in which differential thermal gravimetry (DTG) peaks was observed. These peaks were identified as being due to: dehydration of the physically bound water in the sludge; chemical dehydration of carbohydrates in the sludge; decomposition of hemicellulose, cellulose and proteins in the sludge, and; decomposition of lignin and plastics in the sludge. Addition of chemical additives changed the mass-loss due to chemical dehydration, with the dolomite additive reducing the mass-loss and AS or DAP increasing the mass-loss. AS and DAP also changed the mass-loss kinetics of the decomposition of hemicellulose and cellulose. At temperatures greater than 750°C, the proportion of sludge converted to char was unaffected by the type of additive used. The observed mass-loss data was modelled with a three or four step kinetic mechanism; the calculated kinetic parameters are reported.