The similar conformations and interaction modes of Ac‐DL‐Leu‐Nme2and Ac‐Δ‐Leu‐NMe2molecules in the solid state allow the comparison of their geometrical parameters. The most evident variations are essentially restricted to the α,β‐unsaturated side‐chain which adopts the Z‐disposition. The dimensions of the peptide backbone are much less sensitive to α,β‐unsaturation, with a small shortening by 0.04 Å and 0.02 Å of the N‐Cαand Cα‐C′ bonds, respectively, and an increase by 6° of the N‐Cα‐ C′ bond angle. The ethylenic and amide groups in the Δ‐Leu derivative are far from coplanarity, and a significant electronic conjugat