The electronic structures of vitamins D2 and D3 were studied by He I photoelectron spectroscopy (UPS) and semi-empirical AM1 MO calculations. The analysis of electronic structure reveals two important facts. The vitamin D compounds have electronic structures which resemble steroid hormones and not other vitamins. Although biochemical behaviour and function of the title compounds has long been known to follow the pattern of steroid hormones, we demonstrate in this work that the electronic structures also closely resemble such hormones. The first ionization energies of vitamins D2 and D3 are unusually low (they are comparable to vitamin A derivatives) which raises the possibility that the title compounds may participate readily in various electron transfer processes. This aspect of vitamin D structure had so far been somewhat neglected in biochemical investigations. Also this work demonstrates that the electronic structure may play a significant role in the overall biochemical behaviour of some compounds.