This Test Guideline describes an experimental method using HPLC for the estimation of the adsorption coefficient Koc in soil and in sewage sludge. The adsorption coefficient is defined as the ratio between the concentration of the substance in the soil/sludge and the concentration of the substance in the aqueous phase at adsorption equilibrium.The test substance is injected in the column packed with cyanopropyl solid phase containing lipophilic and polar moieties. As a result of partitioning between mobile and stationary phases the test substance is retarded. The retention time tR is determined in duplicate. The HPLC dead time t0 may be determined by two methods: the determination by means of a homologous series and the determination by inert substances which are not retained by the column. If the actual results show that the log Koc of the test substance is outside the calibration range the test should be repeated using different, more appropriate reference substances.