New techniques are presented for the preparation of iodine derivatives, involving vapour diffusion of iodine. Firstly, in the vaporizing iodine labelling (VIL) technique, a small amount of KI/I(2) solution is enclosed in a crystallization well, with the result that gaseous I(2) molecules diffuse into the crystallization droplets without exerting substantial changes in ionic strength in the target crystals. Once they have diffused into the droplet, the I(2) molecules sometimes iodinate accessible tyrosines at ortho positions. Secondly, when iodination is insufficient, the hydrogen peroxide VIL (HYPER-VIL) technique can be further applied to increase the iodination ratio by the addition of a small droplet of hydrogen peroxide (H(2)O(2)) to the crystallization well; the gaseous H(2)O(2) also diffuses into the crystallization droplet to emphasize the iodination. These techniques are most effective for phase determination when coupled with softer X-rays, such as those from Cu Kalpha or Cr Kalpha radiation. The effectiveness of these techniques was assessed using five different crystals. Four of the crystals were successfully iodinated, providing sufficient phasing power for structure determination.