AbstractIncompatible polymers yield two‐phase binary blends at almost all compositions. In equilibrium, the blends consist of two coexisting phases which have frequently very one sided compositions: Much of one component contains little of the other. The small content of the minor component, the so‐called “partial miscibility”, is important for some properties. But it is difficult to measure. Conventional techniques to determine miscibility gaps which rely on transparency and turbidity fail when the gaps are too wide. More reliably, the composition of the coexisting phases can be extracted from X‐ray or neutron scattering data. A series of blends of poly(methyl methacrylate) (PMMA) and random copolymers SxMMAl−x of styrene and MMA, of which the degree of incompatibility was varied via the copolymer composition x, was studied by small angle neutron scattering. The interactions of the polymer components were measured in homogeneous and demixed blends. The demixed blends yield at high wave vectors a scattering equal to the scattering of the two coexisting phases, superposed. The composition of the coexisting phases was extracted from the slope of the Zimm curve which responds very sensitively even when the compositions are extreme. Miscibility gaps as wide as 99.7% could be determined.