Defines a procedure for the determination of the hydrocarbon oil index in waters by means of gas chromatography. This method is applicable for surface water, waste water and water from sewage treatment plants and permits the determination of a hydrocarbon oil index in concentrations above 0.1 mg/l. This method does not apply to the quantitative determination of the content of volatile mineral oil. However, on the basis of the peak pattern of the gas chromatogram, certain qualitative information on the composition of the mineral oil contamination can be derived.