Abstract: A device for mass transfer between blood and a transfer medium, in particular a gas/gas mixture, includes a chamber through which blood can flow and in which a plurality of mass-permeable hollow fibers of at least one hollow fiber mat, in which the hollow fibers are held at a spacing by way of warp threads, are disposed in the form of a wound or folded hollow fiber package, wherein a transfer medium is able to flow through, and blood is able to flow around, the hollow fibers, and wherein the density of hollow fibers varies locally in the hollow fiber package in the cross-section perpendicular to length of the hollow fibers. Hollow fiber packages and methods of manufacturing thereof are provided in which the hollow fibers are held at a spacing by warp threads, in which the spacing between adjoining hollow fibers is locally increased, in particular compared to a predominantly equidistant spacing between the hollow fibers.