Enamines from 3-methylbutanal and several Pro- and Phe-derived secondary amines were prepared in DMSO-d6, CD3CN, and CDCl3. For the first time, the relative thermodynamic stabilities of these and other enamines were compared, and rapid exchanges of 1-alkenyl groups were demonstrated. Competition experiments showed that the most favored enamines (without significant steric inhibition of resonance) react more rapidly with electrophiles.