A study is carried out to determine the controlling mechanisms of natural convection downward smolder through a porous combustible material, and to observe the effect of buoyancy on the propagation of the smolder reaction. Measurements are performed of the smolder reaction temperature and velocity through polyurethane foam as a function of the smolder reaction location and sample size. Three zones with different smolder characteristics can be identified from the measurements. An initial zone where heat transfer from the igniter results in relatively high smolder velocities. A middle zone with an approximately constant, or slightly increasing, smolder velocity, whose length increases with the foam length, considered to be representative of self-propagating smolder in natural convection. A final zone, near the end of the sample that is characterized by a strong increase in the smolder velocity due to an increase in the flow rate of oxidizer through the sample. A theoretical analysis of the flow field induced by buoyancy through the foam is developed and incorporated to the theoretical model of Dosanjh et al. [I] for opposed forced smolder, and the results of the model are used to correlate the experimental smolder velocity data. The model predicts very well the experimental measurements, and indicates that, for the present experimental conditions, downward smoldering is a process controlled by the supply of oxidizer to the reaction zone.