Dense cellulose acetate, CA, and poly(2,6dimethyl-1,4-phenylene oxide), PPO, membranes were prepared with and without blending a spin probe, TEMPO free radical, and their C-13 solid-state cross-polarization magic angle spinning nuclear magnetic resonance, CPMAS NMR, spectra were taken. It was observed that there was no effect of blending TEMPO on the CPMAS NMR spectra of CA and PPO membranes. It was concluded that there was no chemical or physical interaction between the polymeric material and TEMPO. (c) 2006 Wiley Periodicals, Inc.