A method for concentrating an aqueous caustic alkali produced by a membrane cell process by using a single or multiple effect evaporator system in which the vapor flows in a counter direction to the aqueous caustic alkali flow and the heat recovered from the catholyte circulation line is used as part of the concentration process. In one embodiment, a catholyte heat recovery heat exchanger and flash evaporation chamber are located after the last effect of a multiple effect evaporator system. In another embodiment, the catholyte heat recovery heat exchanger and flash evaporation chamber are located prior to the single or multiple effect evaporator system. In yet another embodiment, the catholyte heat recovery process is used in conjunction with additional heat exchanger processes to further concentrate the final product as desired.