The coolant in a boiling water reactor (BWR) during a cold shutdown usually contains a relatively high level of dissolved oxygen from intrusion of atmospheric air. Accordingly, the structural materials in the primary coolant circuit (PCC) of a BWR could be exposed to a strongly oxidizing environment for a short period of time during a subsequent startup operation. Because there are limited measurable water chemistry data, a well-developed computer code DEMACE was used in the current study to investigate the variations in redox species concentration and in electrochemical corrosion potential (ECP) of components in the PCC of a domestic BWR during startup operations. Our analyses indicated that the dissolved hydrogen level in the reactor coolant at a low power level without steam generation in the core was lower than that at a power level with a minor amount of steam generated in the core. The dissolved oxygen concentrations in the reactor coolant were relatively high and were >500 ppb during startup operations at power levels >2.5%. In the meantime, the concentrations of hydrogen peroxide could be >500 ppb at the core outlet region during startup operations, which renders a strongly oxidizing coolant environment in the entire PCC. The ECPs of structural components in the PCC of the analyzed BWR generally followed the concentration trend of hydrogen peroxide. It was predicted that the coolant environment in a BWR during a plant startup could be highly oxidizing, and the structural components would therefore suffer from a more serious corrosion problem than under operations at the rated power level.