203Hg, 35S or 131I labeled mercury (II) iodide thiocyanate (HgISCN) powders were prepared, respectively. When the powders were exposed to sunlight, some parts of the crystals of the powders were decomposed and the decomposed atoms diffused toward crystal surface. This diffusion velocity was accelerated by thermal treatment of the darkened powders. The velocity is larger in order of 35S, 203Hg, ...