Assessing the fate of endocrine disrupting compounds (EDC) in the environment is currently a key issue for determining their impacts on aquatic ecosystems. The 4 nonylphenol (4-NP) is a well known EDC. It results from the biodegradation of surfactant nonylphenol ethoxylates (NPEO). Biodegradation mechanisms of NPEO are well documented (Giger et al. 2009) but their rate constants have been mainly determined through laboratory experiments. To our knowledge only Jonkers et al. (2005) evaluate NPEO biodegradation rate constants according to field measurements. Their study was carried out in an estuary (salt water) and has to be confirmed for freshwater. This study aims at evaluating the in-situ biodegradation of 4-NP, nonylphenol monoethoxylate (NP1EO) and nonylphenolic acetic acid (NP1EC). Two innovative sampling campaigns were carried out on the Seine River in July and September 2011, from Maisons-Laffitte to Triel-sur-Seine (along a 40km-transect downstream of Paris City). Their results were used for calibrating a sub-model of NPEO biodegradation of the hydrodynamic-biogeochemical model of the Seine River (PROSE, Even et al. 1998). Sampling times were estimated according to the Seine River velocity in order to follow the same volume of water. Simultaneously, during the September sampling campaign, small scale spatial and temporal variabilities of nonylphenolic compounds concentrations were assessed. Biodegradation rate constants of 4-NP, NP1EO and NP1EC between July and September varied greatly. Although the biodegradation rate constants in July were especially high (higher than 1 d 1), those obtained in September were smaller but consistent with the literature. This variability is probably linked to the biogeochemical behaviour of the Seine River. Indeed, the July sampling campaign took place at the end of an algal bloom leading to an unusual bacterial biomass while the September campaign was carried out during an "usual" biogeochemical state. This study provides relevant information regarding biodegradation rate constants of alkylphenols in an aquatic environment. Such data may be very helpful in the future to better understand the fate and transfer of nonylphenolic compounds at the catchment basin scale.