An improved mass transfer heat exchanger in which heat transmission surfaces are more effectively used and having excellent mass transmission characteristics. Heat transmission pipes are arranged vertically in a tank, extending through a partition board dividing the tank into upper and lower chambers. A solution lead-in pipe supplies a high-density solution to be subjected to heat exchange to the upper chamber, and a pump draws a low-density solution obtained by mass exchange out of the lower chamber and delivers a part thereof to the upper chamber. The upper and lower chambers are communicated through a return pipe which delivers to the lower chamber an intermediate-density solution obtained by mixing the high-density solution and a surplus of the low-density solution in the upper chamber. A vapor lead-in pipe supplies vapor to the lower chamber. The intermediate-density solution flows down the heat transmission pipes, passing through gaps between the partition boards and heat transmission pipes.