Abstract: A device for mass transfer between blood and at least one gas/gas mixture, includes first and second chambers through which blood is able to flow and in each of which a respective plurality of mass-permeable hollow fibers are disposed around a respective axially extending core element, wherein a gas/gas mixture is able to flow through, and blood is able to flow around, the hollow fibers, wherein the second chamber follows the first chamber in the blood flow direction, wherein the first and second chambers are disposed next to one another, and in particular disposed spaced apart between the core element center axes thereof, and the two chambers have a connection in an axial end region by which the chamber volumes through which blood is able to flow are connected, and in particular are connected in the direction of the spacing.