Analyses appearing in the literature on combustion of cellulosic fuels assume that the energy transfer in the fuel interior is adequately described by an unsteady state conduction equation provided with an Arrhenius source. It has been discovered experimentally by x-ray photography that even though in certain zones of space and time the above assumption is valid, there exist other zones in which several ‘secondary’ physico-chemical effects of energy evolution and transfer become non-negligible and take a dominating role in the control of the burning rate. The present paper gives a critical summary of the ‘secondary’ processes of energy and mass evolution and transport in wood-burning, in an attempt to formulate them in such a way that a meaningful quantitative handle is possible.