Kinetics of the photolysis of Cu(+2)-amino acid complexes were investigated under sunlight or monochromatic radiation. Under sunlight (latitude 40 deg N) in the absence of dioxygen, the mean half-lives estimated for the photoreduction of the bis-Cu(+2) complexes (CuL2) at pH 8.0 and 25C were 0.55, 1.6, 8.6, 8.8, 45 and 71 days for L = tris(hydroxymethyl)aminomethane (Tris), beta-alanine, L-aspartic acid, L-glutamic acid, glycine, and histidine, respectively. Under monochromatic UV irradiation (310 nm) with dioxygen absent, quantum yields for the photoproduction of Cu(I) from CuL2 were found to be 0.071, 0.098, 0.063, 0.066, 0.025, and 0.0011 for L = Tris, beta-alanine, L-aspartic acid, L-glutamic acid, glycine, and L-histidine, respectively. The effects of pH on the quantum yields were determined for Cu(+2)-beta-alanine and Cu(+2)-Tris complexes. Decreases in quantum yields observed with increasing acidity were attributed to changes in speciation from predominantly CuL2 to less photoreactive complexes, mainly CuL and Cu(+2). In air-saturated solutions, rates and quantum yields for Cu(I) formation were reduced sharply and hydrogen peroxide was formed. Results of the study suggest that photoreactions of Cu(+2) coordination complexes may contribute to its toxicity to aquatic biota.