Summary.Nuclear magnetic resonance spectra have shown that isocil, 5‐bromo‐6‐methyl‐3‐isopropyluracil, exists almost entirely in the ‘keto’ form, i.e. that the hydrogen atom is attached to the nitrogen at position one in the ring rather than to the oxygen at position two. Spectra were obtained in deuterated chloroform to avoid solvent interference. This method may be of value in determining structures of other herbicides where‘keto’‐‘enol’equilibria arc believed to exist.Utilisation de la résonance magnétique nucléaire pour la détermination de la structure de l'isocil (5‐brom