Abstract: A reversible cycle ice making machine provided with a tube-in-tube evaporator having disposed between the tubes thereof an annular chamber through which a refrigerant is adapted to flow and bond a hollow ice column upon the inside surface of the inner tube during the freezing cycle; and during the succeeding defrost cycle, to thaw the bond by a dual acting defrost system.One of the defrost actions of the system is effected by flowing a defrost fluid, such as hot gas, endwise through the annular chamber to cause the refrigerant remaining the chamber from the preceding freezing cycle to be progressively replaced by the defrost fluid while the ice column bond is progressively thawed in the endwise direction. Thus, the length of the chamber containing the defrosting fluid progressively increases as the complementary length containing the remaining refrigerant fluid decreases during the flow of both fluids in tandem and downstream through the chamber.