Further development and testing of the zone method of analysis is described. In the zone method, the temperature and heat-flux distributions in a furnace are obtained by dividing the system into zones, and solving simultaneously energy balances on each zone. The present state of development of the method is discussed, and avenues for further development are considered. A method of including the effect of luminous radiation is described. For this the emissivity ofa cloud of soot particles is represented as the weighted sum of grey gases. It is also shown that the emissivity of a gas-soot mixture can be represented as the weighted sum of grey gases. The validity of this emissivity equation is examined by comparing calculated and measured distributions of radiation intensity through a luminous flame. Good agreement between calculated and measured traverses is found. With luminous flames, large variations in gas-absorption coefficient occur in furnaces.Methods of allowing for variations of gas-absorption coefficient when evaluating radiative “exchange areas” are discussed, and an approximate method is presented. A method of allowing for different types of heat sink in a surface zone is described. Themethod allows for the replacement of the composite surface with an equivalent grey surface having an effective emissivity and temperature. Also, surface energy factors are derived which enable the radiant energy at a surface to be divided in terms of the amount of heat absorbed by each type of sink in the zone. Experimental testing of the zone method of analysis is described. The zone method incorporatingthe abovementioned improvements was used to predict temperature and heat-flux distributions in the IJmuiden furnace. The initial conditions of flow pattern, heat release, and absorption coefficient were obtained from the experimentally measured properties in the furnace. The predictions were compared with measured values of gas temperature and surface heat flux, and good agreement was found. It is concluded from the agreement shown, that the zone method of analysis is a realisticmathematical model which can be used with some confidence for the calculation of heat transfer in furnaces.