The photo-stability of hydrophobic esters of pyridine-3-carboxylic acid and pyridine-3,5-dicarboxylic acid containing 2-ethylhexyl-, octyl-, and dodecyl-substituents which are model extractants of copper from chloride media were studied together with the photo-degradation of their copper complexes. The results indicate that UV light degraded the esters of both pyridine-3,5-dicarboxylic acid and pyridine-3-carboxylic acid. The degree of degradation depended on the structure of the ester, the type of solvent, and the presence of oxygen, water or hydrochloric acid in the solution during exposure to visible or UV-visible light. Photo-reactions between the solvent and the esters were also observed. Copper has a considerable influence on the photo-stability of the esters. (c) 2005 Society of Chemical Industry.