The interaction between urea and tetrabutylammonium acetate was investigated in dimethylformamide/dimethyl sulfoxide solutions using H-1 and N-15 NMR, The chemical-shift behavior of the urea protons is consistent with a urea-acetate hydrogen-bonded complex involving both carboxylate oxygens and the urea hydrogens trans to the carbonyl oxygen with K-assoc = 120 +/- 10. Line shape analysis of the temperature-dependent H-1 NMR spectra show that Delta G(double dagger) for rotation about the C-N bond of urea changes only slightly from 11.0 +/- 0.1 to 11.2 +/- 0.1 kcal/mol on 1:1 molar addition of tetrabutylammonium acetate to a dilute solution of urea. A parallel investigation of the interaction of thiourea with tetrabutylammonium acetate gave a binding constant, K-assoc = 90 +/- 10. The Delta G(double dagger) for rotation about the C-N bond of thiourea was found to increase from 13.5 +/- 0.1 to 14.0 +/- 0.1 kcal/mol on 1:1 addition of tetrabutylammonium acetate to a dilute solution of thiourea in dimethylformamide/dimethyl sulfoxide. Measurements were also made of the self-association of several ureas and of Delta G(double dagger) for rotation about both C(O)-N bonds of 1,1-dimethylurea.