The scavenging activity of diclofenac was estimated by its capability to bleach ABTS radical cation, and ORAC (oxygen radical absorbance capacity) methodology. In addition, GC-MS technique was used to establish if the lactamic derivative of diclofenac was formed as final product in the interaction with peroxyl radicals. Diclofenac bleached ABTS radical cation in a concentration-dependent way. In ORAC assays, diclofenac inhibited the consumption of fluorescein and c-phycocyanin mediated by peroxyl radicals. An ORAC-fluorescein, and ORAC-c-phycocyanin value of 0.3, and 0.004 was estimated, respectively. From the downcurvature observed in Stern-Volmer-like plots, we postulated that the low protection of c-phycocyanin is related to secondary damage of c-phycocyanin by the secondary radical of diclofenac. By GC-MS studies only a 2,6-dichloroaniline derivative was detected as product afterthe reaction with peroxyl radicals.