The dependence of heat release rate on fuel- and environment-based parameters is discussed, The parameters are the combustion efficiency and its convective and radiative components, net heat of complete combustion and heat of gasification, molecular weight of the gasified fuel, equivalence ratio, external and flame heat flux, and surface re-radiation loss. The combustion efficiency of fuels with saturated carbon-hydrogen-oxygen bonds is close to 95% (60% convective, 35% radiative). Increase in bond unsaturation, removal of O atoms and introduction of N and S atoms reduce the combustion efficiency to 88% (50% convective and 38% radiative). Further increase in the bond unsaturation and introduction of benzene rings reduce the combustion efficiency to 72% (32% convective, 40% radiative). Introduction of large numbers of halogen atoms to saturated or unsaturated compounds with or without benzene rings reduces the combustion efficiency to 35%.