Different methods have been used to measure changes in biodegradability/toxicity of aqueous solutions of the pesticide Laition (a commercial formula of methidathion) when it is treated by means of TiO2 photocatalysis: short time biological oxygen demand (BODst) was used to determine the instantaneous biodegradability of the sample; BOD5 was also chosen to determine biodegradability, employing in this case the manometric method; the BOD5/COD ratio was also calculated. Finally, the Zahn-Wellens test was employed to evaluate the long-term biodegradation of the effluents. The inhibition of the respiration of activated sludge in the presence of toxic pollutants was used to test the toxicity of the treated sample. An alternative method based on the decrease of BOD5 of a very biodegradable mixture (glucose + glutamic acid) upon addition of the toxic solution was also employed. Similar trends were obtained with all methods and allowed us to distinguish between two periods: At the beginning of the reaction, there is a decrease in the concentration of dimethoate to reach complete abatement of this pesticide; this produces a nearly complete detoxification of the solution and a very significant increase of biodegradability (BOD5/COD ratio reached values close to 0.5 and important increase of BOD5 and BODst were observed). Beyond this point, slow mineralization is detected, but further improvement of the biodegradability cannot be achieved. (c) 2007 Elsevier B.V. All rights reserved.