The α-heterosubstitutedN, N−diethylacetamides(1–8)cis/gaucheconformations population ratio have been estimated through the analysis of the carbonyl group stretching vibration in the carbon tetrachloride and in chloroform. The substituent steric properties determine thecis/gaucheratio which decreases as the substituent size increases. For most of the studied amides, thegaucherotamer carbonyl band exhibits larger shifts to lower frequencies than in the corresponding α-heterosubstituted propanones1, but for the α-chloro-(4) and for the α-bromo-N,N−diethylacetamide(5) a reverse trend is observed. This behavior can be related to changes in the πC=O C—X Dihedral angle, an hyperconjugative effect being sugges ted to explain the frequency shifts of thegaucherotamers.