The stability limit and spinodal points of a multicomponent polymer system have been studied with the structure factors, S-ij(q), which were obtained using the random phase approximation and incompressibility constraint. Emphasis was placed on the effect of the volume fraction of component A,f(I), in a block copolymer, A-block-B, on the spinodal points for macrophase separation of an (A-block-B)(B-ran-C)/(A-ran-D) mixture. The behaviour of the spinodal point for macrophase separation, and thus the compatibilization effect, depends very much on f(I) for given weight fractions of the two homopolymers (or a homopolymer and a random copolymer). The spinodal points of microphase transitions in addition to those of macrophase separation are also discussed.