Abstract A novel case of polymorphism in platinum(II) complexes is reported. Single crystals of [PtBr₂(2,9-dimethyl-1,10-phenanthroline)] were obtained at room temperature from a dilute dichloromethane/pentane (1:3, v/v) solution of the title compound in the presence of guanosine. The space group resulted to be Pbca [a = 20.4024(3), b = 20.3638(3), and c = 13.9522(2) Å, V = 5796.7(1) ų, Z = 8]. Crystals of the same compound, obtained in slightly different conditions, had already been investigated by X-ray diffraction and found to have P2₁/n space group, [a = 12.155(4), b = 7.933(2), and c = 14.522(2) Å, β = 93.74(2)°, V = 1397.3(6) Å3, Z = 4]. Comparison of the two polymorphous forms reveals huge differences in the crystal packing and the important role of the stacking interactions between phenanthroline ligands.