Apparatus for effecting mass transfer between liquid and gas phases includes a chamber having a tube bundle therein comprised of a plurality of spiral fluted tubes maintained in upstanding relation by transverse tube sheets. Relatively short tubular inserts cooperate with the opposite ends of the spiral fluted tubes in a manner to facilitate passage of liquid downwardly through each tube in the form of a liquid film on the interior spirally fluted tube surfaces, and enable passage of gas upwardly through each tube such that the gas directly contacts the liquid film and effects mass transfer therebetween. Inlet and outlet coolant passages enable coolant to be introduced into the chamber for removing heat of absorption from the external surfaces of the tubes.