A systematic investigation was performed to elucidate the cause of spontaneous ignition of Refuse Derived Fuel (RDF) and Meat Bone Meal (MBM). Heat generation in both RDF and MBM with addition of water liquid and vapor at room temperature was determined by isothermal calorimetry. Compared with water liquid, the heat of wetting by sorption of water vapor at 80% relative humidity and 25 degrees C was larger, which can raise the temperature of RDF and MBM more than 30 and 56 degrees C, respectively. Heat generation due to fermentation occurred and the temperature of RDF and MBM reached or exceeded 80 degrees C after 5 days for RDF and 4 days for MBM at 100% RH. The spontaneous ignition for RDF and MBM results from heat of wetting and fermentation at room temperature and a further exothermic reaction at higher temperature. (C) 2005 Elsevier B.V. All rights reserved.