The invention is directed to an apparatus for determining the proportion of substances having paramagnetic characteristics, such as oxygen, in a mixture of substances. The apparatus has a housing and the mixture of substances is passed along an arrangement of four magnetic field sources by a rotatable cuvette arrangement mounted in the housing. The apparatus is improved in that the effect of external disturbing influences on the penetration of the cuvette by the magnetic field is reduced and so that the measuring characteristics are independent of the position of the plurality of magnetic field generating units. For this purpose, the arrangement of the magnetic field sources is simply disposed at one side of the housing lying opposite to the rotating surface of the cuvette arrangement facing toward the drive shaft. The arrangement of magnetic field sources includes a carrier plate on which the magnetic field sources are accommodated and carry respective measuring field coils. The magnetic field sources face with their end faces toward the cuvette arrangement.