Detailed equilibrium and kinetic studies on axial water ligand substitution reactions of the "lantern-type" platinum(111) binuclear complex, [Pt-2(mu-HPO4)(4)(H2O)(2)](2-), with halide and pseudo-halide ions (X- = Cl-, Br-, and SCN-) were carried out in acidic aqueous solution at 25 degrees C with = 1.0 M. The diaqua Pt(III) dimer complex is in acid dissociation equilibrium in aqueous solution with -log K-h1, 2.69 +/- 0.04. The consecutive formation constants of the aquahalo complex (K-1(X)) and the dihalo complex (K-2(X)) were determined spectrophotometrically to be log K-1(cl), 2.36 0.01 and log '< 2 = 1.47 0.01 for the reaction with Cl- and log K'1' = 2.90 0.04 and 2 = 2.28 0.01 for the reaction with Br-, respectively. In the kinetic measurements carried out under the pseudo-first-order conditions with a large excess concentration of halide ion compared to that of Pt(III) dimer (C-X- >> Cpt), all of the reactions proceeded via a one-step first-order reaction, which is a contrast to the consecutive two-step reaction for the amidato-bridged platinum(III) binuclear complexes. The conditional first-order rate constant (k(obs)) depended on Cx- as well as the acidity of the solution. From kinetic analyses, the rate-limiting step was determined to be the first substitution process that forms the monohalo species, which is in rapid equilibrium with the dihalo complex. The reaction with 4-penten-1-ol was also kinetically investigated to examine the reactivity of the lantern complex with olefin compounds.