Co-combustion of high moisture municipal solid waste (MSW) and high sulfur Thai lignite in a laboratory scale bubbling fluidized bed has been investigated. This paper presents the effects of the mass fraction of a model MSW in the fuel mixture, as against 100% lignite, on the combustion characteristics that may be inferred from temperature distributions, carbon combustion efficiency and CO and CO2 concentrations and on the emissions of major gaseous pollutants, including CO, SO2, NO and N2O. The influence of excess air and secondary air on those characteristics are also discussed. The results show that within the 40% MSW fraction tested, co-combustion is characterized by enhanced combustion of volatiles in the freeboard, a lower bed temperature and a combustion efficiency drop of up to 8%. Co-combustion either increases or reduces CO emission depending on the percentage of excess air. Reduction of up to 7-18% of SO2 is observed at the 40% MSW fraction for the range of excess air and secondary air tested. Both NO and N2O increase slightly with increasing fraction of MSW. Within the range of experimental conditions tested, the optimum condition for -co-combustion in view of combustion efficiency and CO, NO and N2O emissions is: 20% MSW, 40% excess air and 0.2 secondary to total ratio, although the reduction of SO2 is slightly smaller than at other conditions. (C) 2003 Elsevier Ltd. All rights reserved.