Cobalt-60 contained in reactor water primarily causes contamination of boiling water reactor (BWR) out-of-core components. To elucidate the contamination mechanism, the deposition of radionuclides on stainless steels has been evaluated in actual reactor water at a commercially operating BWR. No significant difference was observed between Types 304 and 316L stainless steel. The deposition rate of 60Co was mainly controlled by the growth of oxide films formed on steel. The deposition kinetics of anion 51Cr was very different from that of cations 60Co and 58Co. An Arrhenius temperature dependence was established for the deposition rate of Co between 130 and 240°C.