The aim of this work was to evaluate the effects of UV-visible irradiation on organic carbon concentration, fluorescence intensity and metal transport ability of the natural organic matter (NOM) at fixed pH values and under several types of atmosphere (air, N2, O2). The water samples were obtained from various sampling sites along the Rio Negro (Amazon basin) and the NOM metal transport ability was determined with regard to Cu 2+. As a first step we measured the concentration of total organic carbon (TOC) and its fluorescence intensity before and after an irradiation for a given time. The results demonstrated that when photodegradation happened, the dominant reactions involved O2 and the fluorescent sites were among the first to be altered. Then we determined by spectrofluorometry the complexing capacity (CL) before and after irradiation. Fluorescence and complexing site densities (CL/TOC ratio) increased after a short irradiation time (20 min, i.e. before the photodegradation happened) and decreased after longer irradiation times. Therefore, we conclude that a change of the macromolecular structure activates previously inactive fluorophores and complexing sites prior to photodegradation. ?? 2003 Elsevier B.V. All rights reserved.