We report a systematic study on the redox, spectroscopic, and photophysical properties of a series of [{Mo₆X₈}Y₆]²⁻ (X, Y = Cl, Br, or I. 1–9). All of the [{Mo₆X₈}Y₆]²⁻ clusters show intense and long-lived phosphorescence in both CH₃CN and crystalline phases at 298 K. We found that the emission quantum yields (Φ_em) of 1–9 increase in the sequences X = Cl < Br < I and Y = I < Br < Cl for given Y and X, respectively. The emission lifetimes (τ_em) of the clusters also increase in the sequence Y = I < Br < Cl for given {Mo₆X₈}⁴⁺-core clusters. The present data demonstrate that arbitrary combinations of X and Y in [{Mo₆X₈}Y₆]²⁻ could tune τ_em and Φ_em in the ranges of 85–300 μs and 0.09–0.47, respectively. Both capping (X) and terminal ligand (Y) effects on the photophysical properties of the clusters are discussed on the basis of the energy gap (i.e., emission energy) dependence of the nonradiative decay rate constant.