Bis(8-quinolinolato-N,O)­platinum(II), [Pt(C₉H₆NO)₂], (I), has a centrosymmetric planar structure with trans coordination. The molecules form an inclined π stack, with an interplanar spacing of 3.400 (6) Å. 8-Hydroxy­quinolinium dichloro(8-quinolinolato-N,O)­platinate(II) tetrahydrate, (C₉H₈NO)[PtCl₂(C₉H₆NO)]·4H₂O, (II), is soluble in water and is regarded as the synthetic intermediate of the insoluble neutral compound (I). The uncoordinated 8-hydroxy­quinolinium cations and the monoquinolinolate complexes form an alternating π stack. The origins of fluorescence and phosphorescence in (II) are assigned to the 8-hydroxy­quinolinium cation and the monoquinolinolate–Pt complex, respectively.