Experimental studies on the freezing of porous media show that in case of a fine-grained skeleton matrix a thin transition zone (often called frozen fringe zone) exists between the propagating frozen phase interface and the thawing zone. In this paper a simple analytical criterion for the formation of the freezing zone is presented. The criterion is derived from a quasi-steady model solution, which takes into account moisture diffusion of unfrozen water in both the freezing and thawing zones and neglects convection effects. The model assumes that in the existing temperature range of the freezing zone the thermodynamic equilibrium of unfrozen water can be expressed as a linear function of temperature and that the thermal and mass diffusion coefficients are constant in each zone. The analytical criterion was found to be consistent with experimental results on the freezing zone formation of sandy and silty clay type soils reported in literature.