Based on the known calculated and experimental data on distribution of temperature and gas compositions near the flame front in jets, it is shown that the nature of a change in the total enthalpy during combustion of mixed and unmixed reagents depends on the molecular properties of gases to different extents, but in the same way. Systems with Lewis numbers less than one have a maximum in the enthalpy profile; if the Lewis number is greater than one, a minimum appears near the flame front. Data on heat fluxes associated with thermal conductivity and diffusion are obtained. It has been found that during combustion of unmixed reagents, the heat flux carried by diffusion is an order of magnitude less than the flux carried by thermal conductivity. For mixed reagents, the heat fluxes carried by diffusion and thermal conductivity exceed the heat fluxes in the flame of unmixed reagents.