Different selectivities for photocatalytic oxidation (PCO) of ethanol were observed for two types of adsorption sites on TiO2, and this identification led to the design of a photocatalyst with enhanced selectivity to acetaldehyde, a partial oxidation product. Transient photocatalytic oxidation (PCO), steady-state reaction, temperature-programmed desorption (TPD), temperature-programmed oxidation, and isotope labeling were combined to determine the reactivity of the different adsorption sites. During PCO, weakly bound ethanol preferentially formed gas-phase acetaldehyde, whereas strongly bound ethanol (which decomposes during TPD) primarily produced CO2. Weakly bound ethanol appears to adsorb on sites that are not available for acetaldehyde adsorption. This information, combined with the fact that acetaldehyde decomposes to a strongly bound intermediate during TPD, led to the design of a catalyst that was modified with acetaldehyde TPD products, which were stable during PCO. The TPD products of acetaldehyde preferentially poison the sites where ethanol is strongly bound so that selectivity to acetaldehyde increased during ethanol PCO. Higher selectivity was seen during both transient and steady state experiments, and at 54-60% conversion the ratio of partial to complete oxidation on the poisoned catalyst was five times that on fresh TiO2.