Polyurethane prepolymers prepared from toluene 2,4-diisocyanate, toluene 2,6-diisocyanate, and polypropylene glycol with a ratio between the isocyanate and hydroxyl groups equal to 2 were analyzed by H-1 nuclear magnetic resonance (NMR) spectroscopy in acetone-d(6). Different temperatures and concentrations were used. Toluene 2,4-dimethylurethane and toluene 2,6-dimethylurethane were synthesized and used as model compounds to assign prepolymers signals. Measurements of spin-lattice relaxation time T-1 by "inversion recovery" experiments were carried out on toluene 2,4-diisocyanate, toluene 2,6-diisocyanate, toluene 2,4-dimethylurethane, toluene 2,6-dimethylurethane, and polyurethane prepolymers. Differences in T-1 times were used to interpret prepolymers spectra, by means of the strong observed effect on protons due to the presence of adjacent isocyanate groups.