In the present study the transformation and mineralization of a common aquatic pollutant,, Tramadol (TRA) was investigated for the first time by means of TiO2 photocatalysis. The degradation kinetics for both TRA and total organic carbon (TOC) followed apparent first-order model with rate constants cif k(app) = 15.3 x 10(-2) min(-1) and kapp = 9.7 x 10(-3) min(-1) and half lives (t(1/2)) of 4.5 min and 71.4 min, respectively. The transformation products (TPs) of TRA, were identified by high resolution accurate mass liquid chromatography (HR-LC-MS) suggesting that hydroxylation, oxidation and deallcylation are the main transformation pathways. The reactions were found to occur mainly at the surface of the photocatalyst via surface-bound HO center dot radicals rather than by free diffusion into the homogeneous phase. The potential risk of TRA and its TPs to aqueous organisms was investigated using Microtox bioassay before and during the process. The acute toxicity increased in the first stages and then decreased rapidly to very low values within 120 min of the photocatalytic treatment. The increase in the toxicity is associated with near additive or low synergistic effects between two TPs generated during the process i.e. N-desmethyl and N-oxide tramadol. (C) 2016 Elsevier B.V. All rights reserved.