A short review of the complexities in the specification of physical properties for the solution of thermal problems in freezing and thawing soils is presented. The differential equations governing the conduction of heat, in which latent heat is considered as a heat source, are stated. An equivalent convolution variational theorem is used to construct a two-dimensional finite element model with linear variation of temperature within the element. Two time step idealizations of the equations are described and alternative forms of the latent heat matrix are derived. Some results obtained with the simplest latent heat matrix are presented, which include a problem of complex geometry, to illustrate the versatility of the technique.