The dissolved organic matter in rivers has different origin and its composition can vary both by natural conditions and the anthropic activity that takes place near rivers. This mobile fraction of the organic matter is composed of sub-fractions of different molecular weight and polarity, which confer different complexing capacity for the pollutants and thus a different impact on the environment.The aim of this study was to isolate and characterize the organic matter dissolved in a representative basin of a temperate zone in South America, such as Bio Bio River located in the VIII region of Chile. The material was characterized chemically and spectroscopically after fractionating by molecular weight and polarity.It is concluded that the dissolved organic matter of the Rucalhue sector has a terrestrial origin, with a high aromatic carbon content, in contrast with that from Laja and Concepción, which has a greater anthropic impact. The dissolved organic matters from the sectors with high anthropic activity have larger molecular size and higher organic carbon content and conductivity.