This study was aimed at the degradation of chlortetracycline (CTC) in aqueous solution by direct and indirect photooxidation employing a solar simulator. Ruthenium complexes, tris-(2,2'-bipyridine) ruthenium (II) chloride (Ru3B), and tris-(1,10-phenanthroline) ruthenium(II) chloride (Ru3P) were used as photosensitizers to promote singlet oxygen ((1)[O-2]*) formation in indirect photooxidation. The degradation kinetics of CTC followed a first-order kinetic, and the rate constant and the maximum percentage of degradation decreased by raising the concentration of CTC. This effect was attributed to that the amount of radiant energy absorbed by each CTC molecule decreased by raising the concentration of CTC. Addition of Ru3B produced a 6.98-fold increase in the degradation rate and 3-fold increment in the maximum percentage of degradation in comparison to direct photolysis. This is due to the formation of (1)[O-2]* in the aqueous medium, as corroborated by the finding that a reduction in the level of dissolved oxygen produces a proportional decrease in the removal rate of CTC. High-resolution mass spectrometry studies demonstrated that degradation byproducts result from reduction-oxidation processes and the loss of functional groups, highlighting the hydrolysis and reduction of the primary amide, the formation of double bonds, and the addition of (1)[O-2]* to the aromatic ring of CTC. The presence of humic acid in the solution favored direct photolysis but inhibited the action of photosensitizers. (C) 2015 Elsevier B.V. All rights reserved.