The evaporation of a polar liquid at low pressure and exposed to microwave radiation has been investigated, as an example of an endothermic reaction. A fixed, high column of liquid was irradiated in a waveguide with 2.45 MHz-CW microwave radiation. The temperatures of the gas and the liquid in contact with the interface and the evaporation rate were measured as functions of the non-equilibrium gas pressure and the microwave field.