The phase partitioning of additives in polymer blends has a large impact on the performance of the blend. Therefore, it is necessary to be able to quantify the level of the additives in each phase. A H-1-NMR method is presented to determine the partitioning of additives between the rubber and rigid phases of a high-impact polystyrene (HIPS) material. In one case, a HIPS material was modified with 2,6-di-tert-butyl-4-methyl-phenol (Ionol, CAS# 128-37-OMF) as a stabilizer for both phases. HIPS materials with varying levels of Ionol were melt-blended by extrusion and the total level of additives was determined analytically for these standard materials. The H-1-NMR method was used to determine the level of Ionol in the poly(butadiene) rubber phase. The lonol was found to preferentially partition into the rubber phase with a partition coefficient of about 2. A second example of the same concept, instead utilizing C-13-NMR, involved the analysis of the partition coefficient for both Tinuvin P and Tinuvin 770 (CAS# 2440-22-4 and 52829-07-9), partitioning between the rigid and rubber phases of an ethylene-propylene-diene-modified (EPDM) toughened styrene-ran-acrylonitrile (SAN) copolymer. The partition coefficient was determined to be 0.5 for Tinuvin P and 1.3 for Tinuvin 770.