Natural abundance 15N and 13C NMR spectra of various polyurethanes were measured in trifluoroacetic acid. The polyurethanes were built up from aliphatic diols, polyesters of diols or oligoethylene glycols on the one hand, and from aliphatic α,ω-diisocyanates or aromatic diisocyanates, on the other hand. In addition to the polyurethanes various low molecular weight model compounds, such as ureas, biurets, allophanic acid esters, uretdions and 1,3,5-triazinetriones, were investigated. The substituent effects of phenyl groups in the above mentioned compounds were studied in various solvents. 15N NMR spectra were found to be more useful than 13C NMR spectra for the identification of the various isocyanate derivatives, while 13C NMR spectra have the advantage of a better signal-to-noise ratio.