Copper(I) quantum yields are reported for Cu(II) complexes of aliphatic dicarboxylates in aqueous solution (N-2-purged), based on steady-state illuminations. For Cu(dicarboxylate)(0), the Cu(I) quantum yields at 313 nm (Phi(Cu(I),CuL)) exhibit the following trend (25 degrees C, ionic strength = 0.10 M): malonate (0.15 +/- 0.07) > succinate (0.10 +/- 0.02) (Sun, L.; Wu, C.-H.; Faust, B. C. J. Phys. Chem. A 1998, 102, 8664-8672) > glutarate (0.054 +/- 0.005) > adipate (0.042 +/- 0.004) approximate to pimelate (0.046 +/- 0.009). The systematic decrease in Cu(I) quantum yield observed for these Cu(dicarboxylate)(0) complexes parallels the increasing degree of outer-sphere coordination of the complexes. Free uncomplexed malonate species quench the photoformation of Cu(I) from Cu(malonate)(0) in a way that cannot be explained solely based on the Cu(II) speciation. An interpretation based on the quenching of the intermediate [Cu((CH2C)-C-.(O)O-)(0)] by H(malonate)(-)/H-2(malonalk)(0) is proposed. Evidence is presented for Cu(I) photoformation from Cu(malonate)(0).