Studies were carried out on the photochemical redox reactions of a series of structurally related inner-sphere Cu(II)/dicarboxylate complexes in aqueous solution (N-2-purged), using steady-state illumination. Cu(I) quantum yields (313 nm) of these systems were characterized with respect to the effects of copper(II) speciation, mainly considering the effect of the dicarboxylate structure on the Cu(I) quantum yield. For each solution composition, which corresponds to a unique Cu(II) speciation, the Cu(II)-based molar absorptivity (epsilon(Cu(II)), M-1 cm(-1)) and the quantity Phi(Cu(I)epsilon Cu(II)) (where Phi(Cu(I)) represents the experimental Cu(I) quantum yield) were determined from experiments. This experimental information (epsilon(Cu(II)) and Phi(Cu(I)epsilon Cu(II))) was explicitly combined with the calculated equilibrium Cu(II) speciation (based on critically reviewed thermodynamic data) in a quantitative model to determine molar absorptivities (epsilon(CuL)) and Cu(I) quantum yields (Phi(Cu(I),CuL)) of the Cut complexes, where L represents the dicarboxylate ligand. For Cu(dicarboxylate)(o), the observed relative reactivity of Cu(I) quantum yields at 313 nm (Phi(Cu(I),CuL)) varies by 50-fold and is affected by the dicarboxylate structure as follows : oxalate (0.42) > succinate (0.10) much greater than maleate (0.008). This trend in relative reactivity parallels the trend in expected relative stability of the carbon-centered radicals derived from decarboxylation of the carboxylate (i.e., acyloxyl) radicals formed in the initial photoinduced ligand-to-metal charge (electron) transfer reaction of the Cu(II)/dicarboxylate complex.