An understanding of the development of frost in air-refrigerant evaporators and its influence on heat transfer is important to the optimum design of such equipment. This study constitutes an experimental and theroetical analysis of the mass and heat flow as the frost layer builds up on a tube in crossflow. A closed-cycle air flow system with controlled conditions is used to examine photographically the frost growth on a tube while recording the thermal parameters. A detailed thermal analysis is developed to give an overall prediction of heat transfer variation with time. The analysis is extended to cover thick-finned tubes under conditions in which the air flow rate is a function of the frost buildup.