Cu2O is among the most promising photocatalysts for CO2reduction, however its photocorrosion remains a standalone challenge. Herein, we present an in situ study of the release of Cu ions from Cu2O nanocatalysts under photocatalytic conditions in the presence of HCO3as a catalytic substrate in H2O. The Cu-oxide nanomaterials were produced by Flame Spray Pyrolysis (FSP) technology. Using Electron Paramagnetic Resonance (EPR) spectroscopy in tandem with analytical Anodic Stripping Voltammetry (ASV), we monitored in situ the Cu2+atom release from the Cu2O nanoparticles in comparison with CuO nanoparticles under photocatalytic conditions. Our quantitative, kinetic data show that light has detrimental effect on the photocorrosion of Cu2O and ensuing Cu2+ion release in the H2O solution, up to 15.7% of its mass. EPR reveals that HCO3acts as a ligand of the Cu2+ions, promoting the liberation of {HCO3-Cu} complexes in solution from Cu2O, up to 27% of its mass. HCO3alone exerted a marginal effect. XRD data show that under prolonged irradiation, part of Cu2+ions can reprecipitate on the Cu2O surface, creating a passivating CuO layer that stabilizes the Cu2O from further photocorrosion. Including isopropanol as a hole scavenger has a drastic effect on the photocorrosion of Cu2O nanoparticles and suppresses the release of Cu2+ions to the solution. Methodwise, the present data exemplify that EPR and ASV can be useful tools to help quantitatively understand the solid–solution interface photocorrosion phenomena for Cu2O.