A closed-loop oxygen dosage and electrode potential control system has been designed for slow strain rate testing in high-purity water at controlled electrode potentials. It is used instead of a potentiostat in order to circumvent the experimental difficulty with an IR drop caused by the high resistivity of the environment. By means of this system slow strain rate testing of sensitized American Iron and Steel Institute (AISI) Type 304 (Unified Numbering System [UNS] 530400) stainless steel has been performed in high-purity high-temperature water at different electrode potentials, the occurrence of intergranular stress corrosion cracking being revealed by fractography. In this way the electrode potential below which intergranular stress corrosion cracking does not occur has been estimated; at 200°C a critical potential of −250 mV versus the standard hydrogen electrode (SHE) was obtained.