A novel luminescent platinum(II)-diimine complex, Na 2 [Pt(CN) 2 (dcbpy)]·nH 2 O (H 2 dcbpy = 4,4Ј-dicarboxy-2,2Ј-bipyridine, n = 2 or 5) was synthesized and investigated by using X-ray diffraction measurements and spectroscopy. Complex Na 2 [Pt(CN) 2 (dcbpy)]·2H 2 O was first obtained as a red amorphous solid (1A) that exhibits bright luminescence at 623 nm. After recrystallization, two crystalline forms, the dihydrate form, 1C, and the pentahydrate form, Na 2 [Pt(CN) 2 -(dcbpy)]·5H 2 O (2), were obtained and analyzed by the single-crystal X-ray diffraction method. Upon exposure of the amorphous form 1A to hydrophilic vapours such as those of