Analysis of the gas headspace after photocatalysis by transmission IR spectroscopy reveals that CO produced by TiO2|CotpyP was only formed when 13 CO2 was used as the carbon source (Figure ). Furthermore, in the case of formate, based on previous work using 13 C6glucose with TiO2|CotpyP, C-formate is only formed from photooxidation of 13 C6-glucose, confirming formate's carbon source. These results demonstrate that the cellulose solid fraction, following cellulase pre-treatment, provides a source of suitable electron donors for photocatalytic CO2 reduction. The solar TiO2|CotpyP reforming system was able to convert CO2-to-CO at concentrations of 20 vol% CO2, with an activity drop of only a factor of two despite the five-fold drop in CO2 concentration with respect to 100 vol% CO2 (Figure ). Near stoichiometric amounts of HCOO -to CO/H2