An unexpected complex, triammine(oxalato)platinum(II) in which oxalate anion acts as a monodentate ligand, was isolated in the process of recrystallizing [Pt(NH 3 ) 4 ](C 2 O 4 ). The complex was presumably formed via the loss of one ammonia molecule from [Pt(NH 3 ) 4 ](C 2 O 4 ), followed by the binding of oxalate from the outer coordination sphere. Single crystal X-ray diffraction study revealed that the Pt(II) atom was coordinated on a distorted square by three N atoms of the ammine molecules and one O atom of the oxalate ligand. The crystal packing was stabilized by moderate to strong hydrogen bonds formed between ammine, water molecules and O atoms of oxalate with d(NÁÁÁO) in the range of 2.84(2)-3.29(2) Å , and d(OÁÁÁO) in the range of 2.86(2)-3.37(2) Å . The complex underwent thermal decomposition at 200-231°C in the nitrogen atmosphere, giving rise to Pt, NH 3 and CO 2 .