Hydrogen water chemistry (HWC), aiming at coolant chemistry improvement, has been adopted worldwide for mitigating intergranular stress corrosion cracking in operating boiling water reactors (BWRs). However, a conventional hydrogen injection system employed in this technology was designed to operate only at power levels >30% of the rated power or at coolant temperatures >232°C. This system is usually in an idle and standby mode during a start-up operation. The coolant in a BWR during a cold shutdown normally 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 start-up operation. In this study, the computer code DEMACE was used to investigate the variations in redox species concentration and in electrochemical corrosion potential (ECP) of structural components in the PCC of a domestic BWR during start-up operations with HWC. Simulations were carried out for power levels ranging from 3.8% to 11.3% during start-up operations. Our analyses indicated that for selected power levels with steam present in the core, a higher power level would tend to promote a more oxidizing coolant environment and therefore lead to less HWC effectiveness on ECP reduction. At even lower power levels in the absence of steam, the effectiveness of HWC was more prominent. At a feedwater hydrogen concentration of merely 0.1 parts per million, significant ECP reductions in the PCC of the BWR were observed.