Spectroscopic and kinetic evidence is presented and used to identify and characterize three luminescent, metal-metal-bonded exciplexes. Two long-lived, strongly phosphorescent triplet exciplexes involving the lowest energy triplet excited state of Pt-2(P2O5H2)(4)(4-) (*Pt-2,<(nu)over bar>(max) = 1.95 mu m(-1), Phi(lum) = 0.5, tau = 10 mu s) and Tl-I in aqueous solution at room temperature (*Pt2Tl,<(nu)over bar>(max) = 1.79 mu m(-1), Phi(lum) = 0.5, tau = 10 mu s; *Pt2Tl2,<(nu)over bar>(max) = 1.70 mu m(-1), Phi(lum) = 0.5, tau = 10 mu s) and one weakly fluorescent singlet exciplex (nu(max) = 2.32 mu m(-1), Phi(lum) < 1 x 10(-3)) are formed as a result of direct Pt-Tl bonding. An inner-sphere mechanism for the formation of the two triplet exciplexes is proposed in which a diffusional approach of the ionic reactants results in relativistically influenced Pt-Tl bonding and is accompanied by substantial desolvation effects. Free energies (-30 and -20 kJ/mol), enthalpies (-19 and -10 kT/mol), and entropies (36 and 31 J/(mol K)) for the formation of *Pt2Tl and *Pt2Tl2, respectively, are derived through a novel procedure and, along with the corresponding kinetic values, are consistent with the proposed mechanism. These three exciplexes are the only examples known which form as a result of metal-metal bonding between the reactants.