The effect of selective cross-linking of poly(styrene-co-maleic anhydride) (SMA) in an initially fully miscible blend with poly(styrene-co-acrylonitrile) (SAN) is studied as a function of cross-link density. Site specific C-13-enriched SMA and SAN obtained by total synthesis and CP3 magic angle spinning NMR spin diffusion techniques are used. The cross-linking of SMA results in the formation of a semiinterpenetrating network (semi-IPN) and induces phase separation. The characteristic domain sizes increase roughly linear with the amount of cross-links on a scale of 0-100 nm. The fraction of SAN and SMA that remains homogeneously mixed is also determined, and it is shown to decrease with increasing cross-link density.