Hematite has attracted considerable interest as a photoanode material for water oxidation under visible illumination. Here, we explore the limits of photocatalytic water oxidation activity with iron (III) oxide hydroxide nanocrystals and as a sacrificial electron acceptor ( NHE at ). The sol was prepared by hydrolysis of in boiling 0.002-M HCl solution and confirmed to mainly consist of (akaganéite) particles with 5 to 15 nm diameter. From a 0.01 M aqueous solution, the sol evolves between 4.5 and , depending on pH, light intensity (, 290 to ), , and concentration. The activity increases with pH, and depends linearly on light intensity and photocatalyst amount, and it varies with sacrificial electron donor concentration. Under optimized conditions, the apparent quantum efficiency is 0.19% (at 400 nm and ), and the turnover number is 2.58 based on total . Overall, the efficiency of the photocatalytic system is limited by electron hole recombination and by particle aggregation over longer irradiation times (24 h). Lastly, surface photovoltage measurements on films on fluorine doped tin oxide substrate confirm a 2.15 eV effective band gap for the material.