Competitive photocatalytic oxidation (PCO) of mixtures of Cu(II)-EDTA and Cd(II)-EDTA was studied with variation of molar ratio of these two complexes (1×10{sup}(-4):0, 8×10{sup}(-5)：2×10{sup}(-5), 5×10{sup}(-5):5×10{sup}(-5), 2×10{sup}(-5),0:1×10{sup}(-4) M) and in the pH range of 4-8. PCO rates for each compound can be described using a combined aqueous + adsorbed pathway: -dC/dt= kiC{sub}(aq)/(1+ k{sub}2C{sub}(aq)) + k{sub}(ads)C{sub}(ads). This expression is valid under both noncompetitive and competitive conditions. Differences in rates under competition result from differences in the partitioning of the two species between the TiO{sub}2 surface and the aqueous phase. Total initial complex degradation rates (r{sub}(TT)), obtained by summation of the total destruction rates for Cu(II)-EDTA and Cd(II)-EDTA, were relatively constant at pH 4 and 5 for all ratios. At these pH values, contribution of adsorbed pathways to r{sub}(TT) was important, and rates were similar to those of the aqueous phase pathways. From pH 6 to 8, the degree of adsorption, and thus the adsorbed pathway rate, diminished. Through the adsorbed pathway, no difference in rate constants was found between Cu(II)-EDTA and Cd(II)-EDTA; Cd(II)-EDTA is somewhat more reactive through the aqueous phase pathway.
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