Reverse phase HPLC is performed on analytical columns packed with a solid phase containing long hydrocarbon chains chemically bound onto silica. The chemicals are retained in the column in proportion to their hydrocarbon-water partition coefficient, with hydrophilic chemicals eluted first and lipophilic chemicals last. The HPLC method covers log Pow in the range of 0 to 6, but it can be expanded to cover the log Pow range between 6 and 10 in exceptional cases. The HPLC operation mode is isocratic. The test substance is injected in the smallest detectable quantities in the column. The retention time is determined in duplicate. The partition coefficient of the test substance is obtained by interpolation of the calculated capacity factor on the calibration graph. For very low and very high partition coefficients extrapolation is necessary.