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Investigations were undertaken on a full scale lagoon treatment system receiving effluents from a bleached kraft pulp and paper mill which processed softwoods. The system was examined over four phases, including lagoons, aerobic transport channels and the recipient discharge point to determine the removal efficiency of resin acids during effluent treatment. The total treatment system removed 96% of the influent resin acids. The major compounds remaining after treatment were abietic acid, dehydroabietic acid, and a variety of hydrogenated resin acid transformation products. Each section of the treatment system differed in its ability to remove the major classes of resin acids. For example, the average removal rates for resin acids within the first phase lagoons and of the channel leaving the lagoons were 1.2 and 17 g.kg-1 VSS.day-1, respectively. A pathway for the biodegradation and biotransformation of influent resin acids is proposed on the basis of the observed changes in effluent composition through the treatment system.