Abstract: Disclosed herein is a-variable-area or mass or area and mass ratio species transfer device, one embodiment with a plurality of species transfer masses. At least one actuator is disposed in communication with the species transfer masses, capable of selectively moving one or more of the masses independently of other one or more of the masses into at least contact with a first fluid stream and into contact with a second fluid stream. Yet further disclosed herein is a method for controlling species transfer in a species transfer device. The method includes: selecting an appropriate mass/area ratio between a portion of a variable area or mass or area and mass ratio species transfer device exposed to a higher-temperature fluid and a portion of the species transfer device exposed to a lower-temperature fluid; exposing one selected portion of the species transfer device to the higher-temperature fluid; and exposing another selected portion of the species transfer device to the lower-temperature fluid.