An AOX transport model is used to distinguish natural and anthropogenic sources of organohalogen compounds in river basins. Application of this model to AOX data of the river Rhine basin in the years 1986 to 1991 shows a constant natural AOX contribution of about 15 µg.l-1 to the total AOX flowing into The Netherlands. The anthropogenic discharge has decreased by 70-95% for the river Rhine basin in this six years resulting in a situation that today 30-70% of the total AOX flowing into The Netherlands at Lobith originates from naturally produced organohalogen compounds.