The photophysics of selected tungsten iodide clusters was examined with respect to their role as a photosensitizer for the production of singlet oxygen, O-2(a(1)Delta(g)). We examined all-iodo octahedral clusters, [W6I8(I-6)](2-), and ligand-substituted octahedral clusters, [W6I8(L-6)](2-), in which the ligand, L, occupies the outer apical positions surrounding the cluster core. We also examined a square-pyramidal cluster, [W5I8(I-5)](-), in which the tungsten core was presumably more accessible to diffusional encounter with ground state oxygen, O-2(Chi(3)Sigma(-)(g)). For the compounds examined, we find pronounced cluster dependent changes in the yield of photosensitized O-2(a(1)Delta(g)) production. In particular, although the iodine-encased octahedral cluster, [W6I8(I-6)](2-), is an efficient O-2(a(1)Delta(g)) sensitizer, the pyramidal cluster, [W5I8(I-5)](-), does not make O-2(a(1)Delta(g)) at all. The latter provides fundamental insight into the important case where the sensitizer triplet state is nearly degenerate with the O-2(Chi(3)Sigma(-)(g))-O-2(a(1)Delta(g)) transition energy at 1 eV. Our data indicate that even with near resonance, energy transfer to form O-2(a(1)Delta(g)) will not occur within the (3)sensitizer-O-2(Chi(3)Sigma(-)(g)) encounter pair if other more efficient channels for energy dissipation are available.