Combustion of heavy oil fly ash (HOFA) blended with municipal sewage sludge (MSS) in existing infrastructures may be a sustainable waste management strategy. In this work, the co-combustion of HOFA and MSS with mass ratios from 80/20 to 60/40 is investigated by TGA-FTIR to identify the thermal behaviors (combustion process, ignition and burnout characteristics) and gaseous emissions. Results indicate that co-combustion of HOFA and MSS is better than mono-combustion. MSS induces lower ignition temperatures of the blends, while HOFA tends to decrease the burnout temperatures. The ignition and comprehensive combustibility indices reach the maximum (1.78 x 10(-5) % min(-1) K-2, 2.66 x 10(-8) %(2) min(-2) K-3) in the mixture with HOFA/MSS ratio of 80/20. The co-combustion characteristics of blends are synergistically influenced by HOFA and MSS. Moreover, the CaO originated from MSS contributes to the desulfurization effect to retain the SO2 emission.