Perhaps, the most important technique for elucidation of structure and chain dynamics in polymers is nuclear magnetic resonance (NMR) spectroscopy, and has thus been employed for cross-link density measurements in blends. Early phase-resolved studies using NMR involved the measurement of proton line-widths in solvent-swollen blend specimens. This approach is qualitative at best, as discussed in recent work by this group, in which use of a quantitative alternative was made, as explained below. Using a more direct method, proof of curative migration due to diffusion and solubility in natural rubber and polybutadiene blends has been demonstrated by Klei and Koenig by NMR imaging based upon a Carr-Purcell spin-echo pulse sequence with a gradient magnetic field. The curatives had a higher solubility in polybutadiene and thus resulted in a heterogeneous network.