The H n.m.r. spectra of several 3-(monoalkylamino)crotonic esters indicate that these substances exist either in the liquid state or in solution as a mixture of the geometric isomers (Ia) and (Ib), in which the chelated cis-form is predominant. The isomers can be distinguished by the chemical shifts of the olefinic C-methyl and amino-groups. Unsubstituted 3-aminocrotonic esters appear to exist in the cis-form only, and the NN-disubstituted derivatives in the trans-form.