The biofilm is composed on microalgae and other microorganisms embedded in a polysaccharidic matrix (EPS). In this study, we used the model pesticide diuron, a photosynthesis inhibitor, one of the priority substances in the EU Water Framework Directive, and its main metabolites (DCPMU and DCPU). The objective of this study is to determine the kinetic and equilibrium constants of diuron and its main metabolites in biofilms. Simultaneously, toxic effects were assessed through photosynthetic inhibition, assuming that toxicity would occur once the pesticides are internalized in the algal cells. Our experiments were performed during seven days in an annular bioreactor. Mature biofilm previously grown on artificial substrates was inserted into the bioreactor. The experiment was carried out with a contamination of diuron, DCPMU and DCPU of about 5µg.L-1, for each chemical. Thanks to the bioaccumulation factor (BCF), we could determine that equilibrium biofilm-water was quickly reached for the three tested molecules allowing the calculation of uptake and elimination kinetic constants. Based on a 2-compartment model, a first order kinetic model was fitted with our data. Uptake and elimination constants showed high variabilities which could be linked to a poor fitting of the kinetic model during the initial uptake phase, suggesting more complex accumulation mechanisms. However, after few hours, equilibriums were reached and determined with acceptable data dispersion. Such equilibriums appeared compound-dependent. In the same time, biofilm photosynthesis highlighted three phases: early inhibition (within few hours), probably linked to fast contaminant internalization by cells, a stabilization around a 20% inhibition, a recovery (after few days) which could be related to microorganisms adaptation. Nevertheless, our model based on absorption mechanisms still requires adaptations/improvements to account for others mechanisms likely involved (i.e. diffusion processes and adsorption on EPS). Biofilm is composed with microorganisms (microalgae, bacteria, fungi) and an EPS, containing interstitial spaces which can retain water. Part of the pesticides may have been lost during the biofilm filtration step, thus calling for a more in-depth characterization of their distribution within the biofilm matrix.