The use of photocatalysis to improve the biodegradability of an antibiotic compound, tetracycline (TC) was investigated. The toxicity of TC and its degradation products were also examined. The Sturm test was conducted to assess the biodegradability of by-products formed in the photocatalytic process. The toxicity of tetracycline and its by-products was evaluated using a dehydrogenase inhibition test, which showed a decrease in toxicity during photocatalysis. However, the Sturm test results indicated that, like tetracycline, the by-products are not biodegradable. Possible structures of these by-products were determined using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS). It was found that, during the photocatalytic process, the TC aromatic ring is not opened and the structure of the identified by-products is quite similar to that of tetracycline. A reaction pathway is proposed. (C) 2012 Elsevier B.V. All rights reserved.