A method for the determination of the transition metals Fe, Mn, Ni and Co in the primary water of pressurized water reactors (PWRs) is described and applied to the analysis of water at the Loviisa power plants. Samples are collected via the standard sampling line or an isokinetic line. There is no difference in the results obtained. The water is filtered through a 0.45 µm membrane filter under oxygen-free conditions to separate the soluble and insoluble fractions. The transition metals are concentrated and separated from boric acid using cation chromatography. A quantitative yield is obtained and concentration factors of up to 500 are used. The samples are analysed using inductively coupled plasma mass spectrometry. The detection limits obtained are Fe 2, Mn 0.2, Ni 0.2 and Co 0.02 ng l–1 when a 1 l sample is used.