The compatibility of the two components in semiinterpenetraing polymer networks (semi-IPNs) and corresponding linear blends was studied using the nitroxide spin-label and spin-probe methods. The semi-IPNs and blends were composed of poly(styrene-co-maleic anhydride) (P(S-co-MA)) and poly (methyl vinyl ether) (PVME). Stable nitroxide radicals as reporters were covalently attached to P(S-co-MA) via the maleic anhydride groups ("spin labels") or dispersed in the system ("spin probes"). Electron spin resonance (ESR) spectra were measured at the X band as a function of temperature and composition. The results obtained for the spin probe show that the molecular reporter is not sensitive to variations in the surrounding matrix; therefore no conclusions on the dynamics of the polymer system can be deduced from the ESR spectra. In contrast, the mobility of the spin label is sensitive to the segmental dynamics of P(S-co-MA) : While only one spectral component is detected in the case of pure labeled (P(S-co-MA), two spectral components are observed for all blends and semi-IPNs, indicating dynamically different molecular environments for the P(S-co-MA) segments. The two spectral components are observed even in the miscible region, almost-equal-to 50-60 K below the corresponding cloud-point temperatures. These results reflect fluctuations in the local composition of the mixtures, on a molecular scale <50 angstrom. The introduction of covalent cross-links in the semi-IPNs results in damping of these concentration fluctuations. The correlation times (tau(c)) of the spin labels were related to the dynamics of the systems studied and to macroscopic properties such as T(g).