The rotational mobility of molecules in glassy polymers was investigated by quantitative simulations of electron spin resonance spectra of nitroxide spin probes with molecules of different sizes. The motional models were determined unambiguously when the experimental and calculated spectra coincided within the level of experimental error. Temperature ranges in which different motional models were applicable were determined. Quasi-librational movements and the distribution of rotational mobility were found to be necessary for the qualitative description of spin-probe mobility in glassy polymers. The quantitative and empirical characteristics of rotational mobility were examined. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 102-110, 2011