Abstract: The fractionating apparatus of the invention for the separation of at least two dissolved groups of substances in a solution is of the type wherein an electric field is applied in fractionating chambers separated by semi-permeable membranes so as to generate different migrations by various groups of substances; each fractionating chamber is bounded by a peripheral frame 3 provided with a plurality of strips 23; the two semi-permeable membranes 1 and 2 of the chamber are applied on either side against the faces of said frame and against the faces of said strips; spacer compartments are advantageously provided on either side of the fractionating chambers, said spacer compartments containing an auxiliary solution capable of enhancing by high rate ion exchanges the differences in migrations of the substances to be separated.