The i.r. and ¹H n.m.r. spectra of 3-aminocrotonic esters with primary amino-groups indicate that these substances exist, in the liquid state and in solution, almost exclusively as the isomers with the chelated (Z)-configuration. Some complexities of the i.r. spectra are interpreted in terms of the existence of conformational isomers arising from restriction of rotation around the C–CO₂R single bond. The strength of the hydrogen bond in these compounds is less than that in 3-(alkylamino)crotonates. The i.r. spectra of N-monodeuteriated 3-aminocrotonates also reveal the existence of conformers arising from restriction of rotation around the C–N single bond. The spectra of the N-dideuteriated derivatives show the existence of strong mechanical couplings which affect mainly the ν(CC), ν(C–N). and δ(NH₂) vibration modes. Some very weak bands observed in the ν(N–H) region in certain solvents are attributed to a small concentration of non-chelated molecules.