The 13C and 1H spin-lattice relaxation times of acetone in solutions containing from 0 to 20% poly(methyl methacrylate) have been measured from -55°C to 120°C. The 13C measurements show that except at low temperature and the higher polymer concentrations, solvent rotation is little affected by the presence of polymer molecules. By assuming that the 13C relaxation time is entirely intramolecular in origin, and by progressive deuteration of the acetone, the 1H relaxation time has been separated into intramolecular, acetone-acetone intermolecular and acetone-polymer intramolecular contributions. These results show that up to 10% of polymer or so does not affect solvent diffusion, but for 20% polymer, solvent diffusion is perceptibly slowed down, and the activation energy increased by 50%. The data give no indication of polymer conformational transitions in this temperature range, as suggested by others elsewhere, but this may be because of the very short-range sensitivity of n.m.r. measurements.