The invention relates to a method for separating a mixture containing a) a monoethylenically unsaturated compound that can be obtained by adding two terminal olefins carrying the functional groups necessary for producing the monoethylenically unsaturated compound containing at least two functional groups, or a saturated compound obtained by hydrogenating one such compound, b) a compound that can be obtained by adding more than two of the terminal olefins cited in a) or a compound obtained by hydrating one such compound, and c) a compound which contains a transition metal, is homogeneous in relation to the mixture, and is suitable as a catalyst for producing a monoethylenically unsaturated compound by adding two terminal olefins carrying the functional groups necessary for producing the monoethylenically unsaturated compound containing at least two functional groups. According to said method, the mixture is separated by means of a semi-permeable membrane, obtaining a permeate and a retentate, in such a way that the weight ratio of constituent b) to constituent c) in the mixture supplied to the semi-permeable membrane is smaller than in the retentate.