An experimental study is performed on the magnitude of the different mechanisms by which heat is transferred from the flaming region to the unburnt fuel ahead of the flame for flames propagating horizontally over the surface of a solid fuel. Measurements of the gas velocity field, temperature fields and radiant flux distribution in a particular case of laboratory scale flame spread over a thick fuel are used to determine the magnitude of the heat fluxes ahead of the flame. The results show that, for this particular case, although heat conduction through the solid is dominant, radiation from the flame contributes significantly to the heat transfer process. An analysis of the development of the fire indicates that there is a transition in the mechanisms of heat transfer as the fire grows. While in the early stages of the fire, heat conduction through the solid is dominant, radiation from the flame becomes of increased importance as the size of the fire increases.