In solution, unsubstituted guanine (GH2) is in a tautomeric equilibrium between 1H,7H-keto and 1H,9H-keto forms. Upon coordination with (dien)Pt(II), N9-bound mononuclear complex is preferentially formed. With excess (dien)Pt(II), the dinuclear complex [{Pt(dien)}2(GH-N7,N9)]3+ is formed at basic pH. The X-ray crystal structure analysis of mononuclear complexes [Pt(dien)(GH2-N9)](ClO4)2 x 2.25 H2O (1a), [Pt(dien)(GH2-N9)](Cl)(ClO4) (1b), and the dinuclear complex [{Pt(dien)}2(GH-N7,N9)](ClO4)3 (3) are reported. The N7 linkage isomer [Pt(dien)(GH2-N7)]2+ (2) could not be isolated and was tentatively assigned in solution. In HCl-acidic medium, the dinuclear complex 3 dissociates to form N9 bound mononuclear complex (1) as the major product and presumably the N7 linkage isomer (2) as a minor side product. Species 1 and 3 have been identified by the pD dependence of the 1H NMR guanine resonances. pKa values (calculated for H2O) for deprotonation of the N9 linkage isomer (1a, 1b) are 6.51 +/- 0.01 and 10.53 +/- 0.05 as well as 0.52 +/- 0.07 and 8.55 +/- 0.06 for the dinuclear complex [{Pt(dien)}2(GH-N7,N9)]3+ (3). Ab initio calculations have been performed for [Pt(dien)(GH2-N7)]2+, [Pt(dien)(GH2-N9)]2+ and [{Pt(dien)}2(GH-N7,N9)]3+. They reveal that in gas phase the N7 linkage isomer is more stable than the N9 linkage isomer by DeltaE = 8.4 kcal mol(-1). Reactions with (dien)Pt(II) and cis-(NH3)2Pt(II) were also carried out to study the metal binding behaviour of the ligand, 7-methylguanine (7-MeGH). The complexes [Pt(dien)(7-MeGH-N9)](NO3)(ClO4) (4), cis-[Pt(NH3)2(7-MeGH-N9)2]Cl2 x 2 H2O (5) and cis,cis-[{PtCl(NH3)2}2(7-MeGH-N3,N9)]Cl2 x 3 H2O (6) have been characterized by X-ray crystallography. pKa values for 4 are < 2 and 8.16 +/- 0.04, and for 5 1.86 +/- 0.45 and 8.00 +/- 0.08 (average for deprotonation of two 7-MeGH ligands). Structural peculiarities of N9 platinated guanine ligands are discussed and pKa values of Pt complexes with related guanine nucleobases are compared.