The intensity of a combustion wave moving through a porous, homogeneous fuel array is an important, but poorly defined, concept of fire behavior. Rothermel  suggests the term 'reaction intensity' for the energy-release rate, which is related to the rate of fuel consumption per unit basal area within the combustion zone. A concept of reaction intensity was developed, and the technique used to obtain the intensity distribution within the combustion zone is described. This knowledge is necessary for the characterization of fire intensity in terms that are useful for the prediction of the behavior of a spreading fire. The variable under investigation is the rate of change of mass per unit area (load) of a portion of the array through which the fire spreads. Two methods are employed; choice depends on whether the combustion zone depth is of the same order of magnitude or much greater than the largest dimension of the elemental particle size. Both methods were applied to laboratory excelsior fuel arrays. Results support earlier observations of Rothermel , who reported the existence of an optimum bulk density that gives rise to a maximum reaction intensity. Results from identical fuel arrays also indicate that the reaction-intensity distribution peak broadens and shifts away from the front of the combustion wave as windspeed increases. A similar effect occurs in the absence of wind as fuel compactness increases.