Poly(phenylene oxide) (PPO) polymer's membranes (dense) were prepared by blending spin probes (TEMPO, 5-, 12- and 16-doxylstearic aid) in the casting solution used for the preparation of membranes. It was noticed that the shape and size of the probe influence the ESR spectra of the NO radical in the poly(phenylene oxide)membrane. Unexpectedly, from the shape of the ESR signal it was noticed that of the NO radical of TEMPO in PPO membrane was more mobile than in water media. However, the motion of the NO radical of 16-doxylstearic acid was higher than NO of 5- and 12-doxylstearic acid when the radicals were in the PPO membrane. This could be due to the inductive effect from COOH group. The Hamiltonian parameters of the ESR signal indicated that all the probes were not randomly distributed in PPO membrane, but some probes were in orderly fashion.