The pK(a) values in DMSO of the monoprotic complexes [(L2Pt)(2)(mu-OH)(mu-NMePh)](2+) (4) (L-2 = Ph2PCH2CH2PPh2 (dppe), Ph2PCMe2PPh2 (dppip)) are 11.9 +/- 0.1 (L-2 = dppe) and 13.5 +/- 0.2 (L-2 = dppip) as determined by P-31 NMR equilibrium titration with bases of known pK(a). Complexes 4 were prepared by treatment of [L2Pt(mu-OH)](2)(2+) (1) with N-methylaniline. The oxo complexes [(L2Pt)(2)(mu-O)(mu-NMePh)](+), formed in the equilibrium titration reactions, were independently synthesized in THF by deprotonation of [(L2Pt)(2)(mu-OH)(mu-NMePh)](2+) with NaN(SiMe3)(2) and characterized as NaBF4 adducts. Similar experiments with diprotic [L2Pt(mu-OH)](2)(2+) (L-2 = dppe, Ph2PCH2CH2CH2PPh2 (dppp)) were complicated by exchange processes and were less conclusive, giving pK(a1) < 18 and pK(a2) > 18 in DMSO.