Photo excitation of end-on trans-mu-1,2-peroxodicopper(II) complex [(tmpa)(2)Cu-2(II)(O-2)](2+) (1) (lambda(max) = 525 and 600 nm) and side-on mu-eta(2):eta(2)-peroxodicopper(II) complexes [(NS)Cu-2(II)(O-2)](2+) (2) and [(N3)Cu-2(II)(O-2)](2+) (3) at -80 degrees C in acetone led to one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O-2(2-) + hv -> O-2 + 2e(-)). Interestingly, light excitation of 2 and 3 (having side-on mu-eta(2):eta(2)-peroxo ligation) led to release of dioxygen, while photoexcitation of 1 (having an end-on trans-1,2-peroxo geometry) did not, even though spectroscopic studies revealed that both reactions proceeded through previously unknown mixed-valent superoxide species: [CuII(O-2(center dot-))Cu-I](2+) (lambda(max) = 685-740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer to yield an "O-2-caged" dicopper(I) adduct, Cu-2(I)-O-2, and a barrierless stepwise back electron transfer to regenerate 1 occurred. Femtosecond laser excitation of 2 and 3 under the same conditions still led to [Cu-II(O-2(center dot-))Cu-I](2+) intermediates that, instead, underwent O-2 release with a quantum yield of 0.14 +/- 0.1 for 3. Such remarkable differences in reaction pathways likely result from the well-known ligand-derived stability of 2 and 3 vs 1 indicated by ligand-Cu-II/I redox potentials; (N5)Cu-I and (N3)Cu-I complexes are far more stable than (tmpa)Cu-I species. The fast Cu-2(I)/O-2 rebinding kinetics was also measured after photoexcitation of 2 and 3, with the results closely tracking those known for the dicopper proteins hemocyanin and tyrosinase, for which the synthetic dicopper(I) precursors [(N5)Cu-2(I)](2+) and [(N3)Cu-2(I)](2+) and their dioxygen adducts serve as models. The biological relevance of the present findings is discussed, including the potential impact on the solar water splitting process.