The correlation times, tau(R), for rotation in the slow-motion regime of nitro?;ide free radicals sited at chain-end positions on poly(methyl methacrylate) (PMMA) in the pure, unblended state and in a 1:1 (w/w) freeze-dried blend with unlabelled poly(2-ethyl hexyl methacrylate) (PEHMA) were determined by computer simulation of the slow-motion component of the composite electron spin resonance (e.s.r.) spectrum. Comparison of tau(R) values indicates that the rubbery PEHMA apparently plasticizes the rotation of nitroxide-labelled PMMA chain ends which are expected to be located in the pure glassy PMMA phase. Although the spin-label studies seem to suggest that PMMA and PEHMA are partially miscible, dynamic mechanical thermal analysis (d.m.t.a.) and phase-contrast microscopy prove that PMMA-PEHMA blends comprise two separate phases which are essentially immiscible. The conclusions from the e.s.r., d.m.t.a. and phase-contrast microscopy experiments can be reconciled in terms of a wide-diffuse interphase between the bulk PMMA and PEHMA phases.