The photophysics of selected tungsten iodide clusters was examined with respect to their role as a photosensitizer for the production of singlet oxygen, O2(a1Δg). We examined all-iodo octahedral clusters, [W6I8(I6)]2-, and ligand-substituted octahedral clusters, [W6I8(L6)]2-, in which the ligand, L, occupies the outer apical positions surrounding the cluster core. We also examined a square-pyramidal cluster, [W5I8(I5)]-, in which the tungsten core was presumably more accessible to diffusional encounter with ground state oxygen, O2(X3Σg-). For the compounds examined, we find pronounced cluster-dependent changes in the yield of photosensitized O2(a1Δg) production. In particular, although the iodine-encased octahedral cluster, [W6I8(I6)]2-, is an efficient O2(a1Δg) sensitizer, the pyramidal cluster, [W5I8(I5)]-, does not make O2(a1Δg) at all. The latter provides fundamental insight into the important case where the sensitizer triplet state is nearly degenerate with the O2(X3Σg-)-O2(a1Δg) transition energy at 1 eV. Our data indicate that even with near resonance, energy transfer to form O2(a1Δg) will not occur within the 3sensitizer-O2(X3Σg-) encounter pair if other more efficient channels for energy dissipation are available.