The buildup of radioactivity was measured on Type 304 stainless steel and carbon steel coupons exposed to reactor water in a test loop, which operated for ∼3 yr in a commercial boiling water reactor. Most of the activity buildup could be ascribed to 60Co. After an initial rapid increase, the activity buildup slowed down and kept increasing linearly up to 10 000 h of effective exposure to reactor water. No correlation was found between the 60Co activity in reactor water and the 60Co activity on the coupons. Preoxidation of Type 304 stainless steel coupons reduced the initial activity buildup. Carbon steel picked up much less activity than stainless steel during the tests. Measurements of the activity buildup after filtration of the primary coolant indicated that soluble impurities rather than particulates are primarily responsible for the radiation buildup phenomena. Increasing the coolant velocity also resulted in a reduction of the initial activity buildup on the coupons.