The deactivation process for the excited-state decay of hexanuclear rhenium(III) sulfide and selenide clusters, [Re6S8](2+) and [Re6Se8]2(+), has been identified from temperature-dependent photophysical measurements and electronic structure calculations. Plots of In k(nr) vs T-1, where k(nr), is the nonradiative decay rate constant, yield after allowing for activation energies for nonradiative energy dissipation ranging from 1221 to 1933 cm(-1) I, low-temperature tunneling,. Nonlocal density-functional calculations, with varying proportions of Hartree-Fock exchange, well approximate the experimental 9 K emission maximum of the representative compound (Bu4N)(4)[Re6S8Cl6]. The adiabatic potential energy surface of the luminescent, lowest energy triplet excited state is substantially fixed by symmetry, and the calculations provide quantitative characterization of its features. Notably, the experimental activation energies for a. D-4h(B-3(1g)) - O-h((1)A(1g)) excited-state decay process have the same magnitude as zero-point corrected energies of well-defined PES local maxima. The results herein further contribute to rhenium chalcogenide Clusters' stature as the most thoroughly characterized luminescent metal-metal bonded clusters known.