The analysis of PCB's and dioxins in water by activation with high energy photons is described. The substances were traced by heterochlorine. PCB and dioxins were separated selectively in the presence of a biphenyl-dibenzofurane mixture as a collector by extraction with hexane. Polar compounds were rinsed out, and chloroalkyles were evaporated. After exposure of the collector to 30 MeV bremsstrahlung it was decomposed following the Schöniger procedure. The chemical yield was measured through activity counting of11C separated as11CO2 from the collecting agent, in comparison with a reference material. The chlorine yield was determined gravimetrically using a stoichiometrically well-determined reference material. A few nanograms of chlorine per millilitre are detectable whereby primary samples of 1 l or more may be treated. The described procedure can be recommended as a reference technique for analyses via chromatography/mass spectrometry. Simultaneous analyses of other halogens are planned.