The invention relates to a method for producing a device for a mass transfer between two fluids, wherein at least one hollow-fiber mat (9) is wound on an at least partly hollow core assembly (1, 1a, 1b, 2), and the formed coil is inserted into a housing (10). The assembly of the housing (10) and the coil is then sealed (10), in particular potted, with a sealant at the opposing axial ends in the regions between the hollow-fiber ends and the housing. The core assembly (1, 2) is made of at least two axially adjacent core parts (1, 1a, 1b, 2) arranged one behind the other, at least one (1, 1a, 1b) of which has a hollow design, and the core parts (1, 1a, 1b, 2) are kept in specified axial positions relative to each other, in particular at a distance to each other, by means of at least one aid element (7) at least over the period of the sealing process and preferably over the period of the winding process as well. After the sealing process and the removal of the at least one aid element (7), at least the axially end-face core parts (1, 1a, 2) are kept in their relative positions to each other by means of the sealant. The invention also relates to a coil, a core assembly, and a core system.
