Burning rate of a carbon-cement sphere in a hot air stream was investigated both analytically and experimentally. Under diffusion-controlled conditions, the interface between the reacted shell and the unreacted core could very distinctly be recognized and its radius could be determined from the temperature recording chart at any moment. Based on non-isothermal analysis of an unreacted-core model, the data could be explained satisfactorily by the effective diffusivity, D_e, which showed good correspondence with the permeability coefficient, k.