The biodegradation of two substrates and the ability of dissolved organic matter (DOM) to influence benzo[a]pyrene bioavailability as DOM biodegradation progressed were monitored in reactors. Substrates were composed of algae extracts and an artificial substrate that mimics raw wastewater, which were considered to be autochthonous and anthropogenic allochthonous models for DOM, respectively. The soluble microbial products formed during biomass activity were also studied. The aromaticity of DOM was investigated with specific ultraviolet absorbance. Partitioning coefficients between DOM and benzo[a]pyrene, K(DOC)(biol), were biologically determined by means of 4-h bioaccumulation experiments on Daphnia magna. Parent and degraded substrates always significantly reduced the bioaccumulation of benzo[a]pyrene at environmental DOM concentrations. Soluble microbial products also significantly affected the benzo[a]pyrene bioaccumulation. K(DOC)(biol) ranged between 2 x 10(4) and 4 x 10(5) L/kg. As the artificial wastewater biodegraded, DOM aromaticity increased, as did K(DOC)(biol). During the biodegradation of algae extract DOM, K(DOC)(biol) increased, whereas their aromaticity slightly decreased.