A pulse sequence has been implemented for the determination of domain sizes in C-13 enriched organic materials with two-dimensional high-resolution magic angle spinning (MAS) NMR correlation spectroscopy. The method integrates the high resolution of C-13 MAS for selection and detection with an efficient polarization transfer across domain boundaries via strong H-1 dipolar interactions. A one-dimensional version with spectral editing allows fdr improved efficiency with respect to measurement time. With this MAS NMR and labeling approach heterogeneity at length scales from 1 to 100 nm can be determined for systems without long-range order. As an example, proton spin diffusion times up to 500 ms were used to study the morphology of a phase-separated semi-interpenetrating network of poly(styrene-co-acrylonitrile) and cross-linked poly(styrene-co-maleic anhydride). From the spin diffusion behavior a characteristic domain diameter d = 63 +/- 4 nm was calculated.