We have studied the effects of the oxidizing species on the cobalt radioactivity buildup behavior in boiling water reactors (BWRs) using both experimental results and existing literature data. The oxidizing species used to simulate the normal water chemistry (NWC) condition of BWRs were 200 ppb dissolved oxygen or 200 ppb hydrogen peroxide accompanied by 100 ppb dissolved oxygen. We found that the amount of cobalt deposited on stainless steel specimens in the oxygen-based water chemistry (200 ppb dissolved oxygen) was larger than that in the hydrogen peroxide–based water chemistry (200 ppb hydrogen peroxide and 100 ppb dissolved oxygen). The rate of cobalt deposition in the former chemistry was more than four times larger than that in the latter chemistry. This difference in cobalt deposition behavior can be attributed to two properties of oxides: surface morphology and composition. The film formed in the oxygen-based environment was less dense than the film formed in the hydrogen peroxide–based environment. Regarding the chemical constituents of the oxides, iron chromite is considered to be a major spinel-type oxide formed in oxygen-based environments. Furthermore, some literature data suggest that in hydrogen peroxide–based conditions, hematite-rich oxides are formed instead of magnetite-rich films, which are observed in oxygen-based conditions. These are likely reasons why the stainless steel specimens incorporate more cobalt radioactivity in the oxygen-based environment than in the hydrogen peroxide–based environment. The cobalt buildup behavior after switching from NWC to hydrogen water chemistry (HWC) is also affected by the oxidizing species used to simulate NWC; exposure to hydrogen peroxide–based NWC conditions tends to suppress the cobalt radioactivity buildup after switching from NWC to HWC compared to exposure to oxygen-based NWC.