The structures, optical bandgap sizes, and photocatalytic activities are described for three copper-octamolybdate hybrid solids prepared using hydrothermal methods, [Cu(pda)](4)[beta-Mo8O26] (I; pda = pyridazine), [Cu(en)(2)](2)[gamma-Mo8O26] (II; en = ethylenediamine), and [Cu(o-phen)(2)](2)[alpha-Mo8O26] (III; o-phen = o-phenanthroline). The structure of I consists of a [Cu(pda)](4)(4+) tetramer that bridges to neighboring [beta-Mo8O26](4-) octamolybdate clusters to form two-dimensional layers that stack along the a axis. The previously reported structures of II and III are constructed from [Cu-2(en)(4)Mo8O26] and [Cu-2(o-phen)(4)Mo8O26] clusters. The optical bandgap sizes were measured by UV-vis diffuse reflectance techniques to be similar to 1.8 eV for I, similar to 3.1 eV for II, and similar to 3.0 eV for III. Electronic structure calculations show that the smaller bandgap size of I originates primarily from an electronic transition between the valence and conduction band edges comprised of filled 3d(10) orbitals on Cu(I) and empty 4d(0) orbitals on Mo(VI). Both II and III contain Cu(II) and exhibit larger bandgap sizes. Accordingly, aqueous suspensions of I exhibit visible-light photocatalytic activity for the production of oxygen at a rate of similar to 90 mu mol O-2 g(-1) h(-1) (10 mg samples; radiant power density of similar to 1 W/cm(2)) and a turnover frequency per calculated surface [Mo8O26](4-) cluster of similar to 36 h(-1). Under combined ultraviolet and visible-light irradiation, I also exhibits photocatalytic activity for hydrogen production in 20% aqueous methanol of similar to 316 mu mol H-2 g(-1) h(-1). By contrast, II decomposed during the photocatalysis measurements. The molecular [Cu-2(o-phen)(4)(alpha-Mo8O26)] clusters of III dissolve into the aqueous methanol solution under ultraviolet irradiation and exhibit homogeneous photocatalytic rates for hydrogen production of up to similar to 8670 mu mol and H-2.g(-1) h(-1) a turnover frequency of 17 h(-1). The clusters of III can be precipitated out by evaporation and redispersed into solution with no apparent decrease in photocatalytic activity. During the photocatalysis measurements, the dissolution of the clusters in III is found to occur with the reduction of Cu(II) to Cu(I), followed by subsequent detachment from the octamolybdate cluster. The lower turnover frequency, but higher photocatalytic rate, of III arises from the net contribution of all dissolved [Cu-2(o-phen)(4)(alpha-Mo8O26)] clusters, compared to only the surface clusters for the heterogeneous photocatalysis of I.