Vapochromic behavior of the crystal of dicyano(2,2'-bipyridine)platinum(II), [Pt(CN)(2)(bpy)], has been investigated structurally and emission spectroscopically. Red Form with an infinite Pt...Pt chain transforms to Yellow Form reversibly by taking up water vapor. The crystal structure of Yellow Form ([Pt(CN)(2)(bpy)].H2O) was determined by X-ray analysis: the square-planar complexes form the inclined stack with short and long Pt...Pt distances (3.3279(3) and 4.6814(3) Angstrom). The crystal water makes hydrogen bonds with CN- ligands connecting the stacks. The transformation is accompanied by the sensitive luminescence change as well as the color change, which reflects the change of the Pt...Pt electronic interactions. The sensitive vapochromism for this system is attributable to the slight deformation of the stacking structure induced by water molecules.