The translational diffusion coefficients of a small paramagnetic molecule (the deuteriated nitroxide PDTEMPONE) as tracer were measured in solutions of polystyrene (PS) in toluene and dimethylformamide (DMF) and in cross-linked polystyrene (cPS) networks swollen by the same solvents, using two-dimensional spatial-spectral electron spin resonance imaging (2D ESRI). PS solutions containing 30, 40, and 50 w/w % polymer were examined in toluene at 300, 305, 310, and 315 K and in DMF at 300 K. PS networks prepared with 0.5, 1.0, 2.5, and 5.0 mol % divinylbenzene as cross-linker were swollen in toluene or DMF and studied at 300 K. Two-dimensional images describing the distribution of the diffusant in the sample were reconstructed from a complete set of projections obtained as a function of the magnetic field gradient applied along the symmetry axis of the sample. The concentration profile of the diffusant at a given time was obtained by integrating absorption ESR spectra in slices perpendicular to the spatial axis, which was divided into 256 points. These profiles were obtained as a function of time, and the diffusion coefficients were determined by simulating the experimental diffusion profiles using Fick’s model of diffusion. The diffusion coefficients were found to depend on the solvent, temperature, and PS concentration in the solutions and are significantly reduced by cross-linking in the swollen networks. The temperature dependence of the diffusion coefficients shows an Arrhenius behavior, and their dependence on the polymer concentration in the solutions is consistent with the free volume theory. Results from this study were compared with those obtained for solvent selfdiffusion and diffusion of other tracers in PS solutions and swollen networks.