During thawing of foods in humid air, the ambient water vapour undergoes a phase change at the food surface. The surface phenomena during thawing were formulated into a mathematical model consisting of six stages: frost formation, frost consolidation, transition, condensation, evaporation and drying. This model was coupled with another model developed for heat conduction with phase change inside food during thawing. A numerical solution scheme for the combined thawing model was formulated and a computer program was developed for implementation. A number of experiments were conducted on the thawing of Tylose slabs to compare with the predictions of the thawing model. The predictions of thawing time agreed well with the experimental results. The model was ussed to simulate the growth and decay of frost, ice and water layers on the surface of the food and also to study the effect of temperature and humidity of air, convective film heat transfer coefficient, and shape and size of food on thawing time.