Complex Na2[Pt(CN)2(dcbpy)]·2H2O (1) was first obtained as a red amorphous solid (1A). Elemental analysis of 1A clearly shows that the complex contains two water molecules per Na2[Pt(CN)2(dcbpy)] unit. This red amorphous form is stable in dry air, but the colour gradually changes from red to yellow in humid air. In order to clarify the origin of this colour change, powder X-ray diffraction (PXRD) measurements were conducted on the amorphous red solid 1A upon exposure to various vapours. Figure 1 shows the PXRD patterns of 1A exposed to several kinds of vapours over a period of two weeks. Before exposure, the PXRD pattern was very broad, indicating that 1A is an amorphous solid. After exposure to hydrophilic vapours such as MeOH and acetone, the pattern dramatically changed to one with many sharp peaks, which is characteristic of crystalline solids. Surprisingly, all the patterns obtained for the samples exposed to hydrophilic vapours were almost identical to each other and they essentially corresponded to those obtained for the crystal of the dihydrate form, 1C. The transformation was almost completed within 1 d in a saturated MeOH vapour atmosphere (Figure S1). These results suggest that the amorphous solid 1A changed to the crystalline phase 1C (the structure of 1C is discussed below) upon exposure to the hydrophilic vapours. It should be noted that the amorphous solid 1A was never converted into the crystalline 1C by raising the temperature up to 410 K (Figure S2). In the 1H NMR spectra of the samples exposed to the vapour for two weeks, vapour molecules were hardly detected (Figure S3). In addition, IR spectra measured for samples exposed to acetone vapour suggested that the acetone vapour was not included in the solid (Figure S4). Thus, this structural transformation was triggered by exposure to the vapours, but the inclusion of vapour molecules into the solid did not occur. On the other hand, upon exposure to hydrophobic vapours such as Et2O and CHCl3, no change was observed, suggesting that the amorphous–crystalline transformation from 1A to 1C is probably related to the hydrophilic nature of the vapour. It should be emphasized that the amorphous solid 1A has very poor solubility not only for hydrophobic solvents but also for hydrophilic ones, except for water. When exposed to water vapour, the PXRD pattern of 1A was also converted to a pattern similar to that calculated for the crystal structure of 1C, but other peaks that could not be indexed were also observed. Since the PXRD pattern of 1A immersed in a small amount of water was almost identical to the pattern calculated from the structure of pentahydrate form 2 (Figure S5), the pattern may be due to other transformations, for example 1C to 2. The obtained crystalline solid of 1C is thermally stable and 1C could not be converted back to 1A even under vacuum at a high temperature around 373 K. Thus, this vapour-induced amorphous–crystalline transformation is irreversible in the solid state. As described in the introduction, many vapochromic PtII–diimine complexes have been reported so far, and their vapochromic behaviour originates primarily from the adsorption of vapour molecules, resulting in the reversible colour change. In the case of the transformation from red amorphous 1A to yellow crystalline 1C, the structure was transformed irreversibly from 1A to 1C by exposure to hydrophilic vapour, but the adsorption of vapour did not occur except for water vapour. To the best of our knowledge, such ordering induced by vapour molecules has not been reported for the system based on the PtII–diimine complex so far.