We report here a STXM study of the origin of the reduced onset potential determined from PEC measurements of nanorod hematite photoanodes synthesized by hydrothermal growth from aqueous solutions (ACG). STXM spectromicroscopy successfully quantified the chemical composition of the heterogeneous hematite nanorods and determined the nature of the hematite photoanode/electrolyte interface. A proton-rich phase at the photoanode/electrolyte interface was detected by STXM. This surface phase decreases the potential needed for photocurrent generation, i.e. the onset potential for the water splitting process.