A copolymer of styrene containing less than 5 mol % of chain units spin-labeled by attachment of the nitroxide via a short tether has been synthesized. ESR spectra of dilute toluene solution of the copolymer have been obtained using 9 and 250 GHz ESR. Parameters characterizing polystyrene segmental rotational diffusion in toluene solution over a broad temperature range have been determined from nonlinear least-squares fits of theoretical ESR spectra to the experimental ESR spectra. The model used was that of relatively fast internal rotation of the nitroxide about its tether, with slower polymer chain segmental motion. Together they lead to effective rotational diffusion with an anisotropic diffusion tensor. In addition, constraints in the form of an orientational potential restrict the range of angles over which this diffusion occurs relative to the polymer backbone, and the latter is assumed to reorient on an ultraslow time scale. This is referred to as a model of microscopic order but macroscopic disorder (MOMD). Rates for the slower polymer chain segmental motion (from the 250 GHz spectra) ranged from 3.6 × 10 8 s -1 at 311 K to 0.15 × 10 8 s -1 at 215 K, with a substantial orientational potential of about 2kT over this temperature range. Although there was reasonable agreement between the results obtained at 9 and 250 GHz, there were systematic discrepancies such that the orienting potentials obtained from the 250 GHz spectra were about twice (or more) those from the 9 GHz spectra, and the rotational diffusion tensor components from the 250 GHz spectra were at least twice those from the 9 GHz spectra. This implies a breakdown of the MOMD model for the 9 GHz spectra presumably due to their sensitivity to the slower overall tumbling motion at this low spectral frequency. For the faster "time scale" of the 250 GHz spectra, such a slow motion is "frozen out", rendering these spectra consistent with the MOMD model. Nevertheless, the results at both frequencies yielded a common activation energy, E exp ) 20.7 ( 1.5 kJ/mol, which, when corrected for the viscous flow contribution, yielded an Ea ) 11.9 ( 1.5 kJ/mol, which is in good agreement with recent results from fluorescence studies.