Abstract A series of new platinum(II) complexes with diethyl (2-dqmp) and monoethyl (2-Hmqmp) 2-quinolylmethylphosphonates have been prepared and studied. Both organophosphorus ligands by reaction with [PtX 4] 2− (X=Cl, Br) form either the molecular or ionic complexes depending on the acidity of the reaction solution. Dihalide adducts, trans-[PtL 2X 2] (L=2-dqmp, 2-Hmqmp), with N-bonded ligand through the quinoline nitrogen were obtained in the neutral medium, while under acidic conditions at pH<3 were isolated the ion-pair salt complexes, [LH] 2[PtX 4], containing the protonated quinoline ligand as cation and tetrahaloplatinate complex as anion. In addition, 2-Hmqmp at pH∼3.5 forms quinolinium hexahalodiplatinum salt complexes, [2-H 2mqmp] 2[Pt 2X 6], while the chelate complex, [Pt(2-mqmp) 2]·2H 2O, with N,O-bonded ligand through the quinoline nitrogen and the deprotonated phosphonic acid oxygen was obtained at pH>6. The new complexes were characterized on the basis of elemental and thermogravimetric analyses, conductometric measurements, and by infrared and 1H NMR spectral studies. As a preliminary assessment of their biological activity, complexes were evaluated for their in vitro cytostatic activity in an epidermoid human carcinoma (KB) and murine leukemia (L1210) cell lines. The results obtained were compared with those obtained for the corresponding Pd(II) complexes.