The transient heat and mass transfer in a moist porous medium adjacent to a cylindrical heat source is analyzed in order to characterize the thermal stability of the medium. In the context of this paper, thermal instability occurs in a moist porous medium as a result of significant drying due to excessive thermally induced moisture movement. A dry zone is created which propagates into the medium and thereby inhibits dissipation of heat from the source. The drying of the porous medium adjacent to the heat source is predicted to occur in two distinct stages. During the first stage the rate of moisture movement initially decreases until a “critical moisture content” is reached. Thereafter the drying rate generally increases until complete drying of the material in the vicinity of the heat source has occurred. The value of the critical moisture content is found to be essentially independent of the strength (heat transfer rate per unit length) of the heat source. The parameters which most significantly influence the transport processes are identified and correlated on the basis of numerical solutions of the governing equations. The critical moisture content and critical heat flux are defined and used to quantify thermal stability limits. The correlations are validated through comparison with experimental measurements made on a native soil. The thermal stability model developed in this work establishes the relationship between the thermal stability limits for large-diameter and small-diameter heat sources. From this relationship the thermal stability associated with large-diameter sources can be accurately predicted from experimental measurements using small-diameter sources and small samples.