The structure and dynamics of the Dess-Martin periodinane, a I(V) iodobenzene compound widely used in organic synthesis as a mild oxidant, were studied by a combined (17)O NMR and DFT calculations approach. The results show that a degenerate [1,3] sigmatropic shift of iodine between the two oxygen atoms of each of the three acetoxy groups occurs in solution. The energy barrier of this process depends on the position of the acetoxy group with respect to the iodoxolone ring and is much lower than the energy barrier observed for similar I(III) compounds.