Au/α-Fe2O3 catalyst was synthesized using a modified co-precipitation method to generate an inverse catalyst model. The effects of introducing CO2 and H2O during preferential oxidation (PROX) of CO were investigated. The goal of this work was ≥99.8% CO conversion at 80°C. There was an increase in the conversion at all temperatures with the introduction of CO2 and 100% of the CO was converted at the target temperature of 80°C for any amount of CO 2. Furthermore, there was an increase in conversion to 100% for water fractions ranging from 3% to 10%. Finally, for realistic conditions of (bio-)fuel reforming, 24% CO2 and 10% water, 99.85% conversion was achieved. A long-term test of 200 h showed no significant deactivation of the catalyst at a temperature of 80°C in presence of 24% CO2 and 3% water. The mechanism for PROX is not known definitively; however, current literature believes the gold particle size is the key. In contrast, we emphasize the tremendous role of the support particle size.