Jones and Mason claimed a number of years ago (1) that rearrangement of cis-olefinic acyl azides gave trans-olefinic isocyanates. However, several other workers have found that rearrangement of an acyl azide group attached to an optically active carbon gives configurationally identical products, a discovery which apparently conflicts with Jones and Mason's findings. We have reexamined the rearrangement of cis-crotonyl azide with stringent checks at each reaction step and found perfect retention of geometry in the product. Details of this finding are discussed.