Photocatalysis is an effective method of oxidizing or decomposing organic contaminants present in low concentrations in the air at room temperature. The rate of photocatalytic decomposition (PCD) of formic acid on 0.2% Pt/TiO2 was studied using transient isothermal reaction and temperature programmed desorption. Transient photocatalytic decomposition shows that water co-adsorbed with formic acid does not significantly affect the rate of decomposition. Water that is weakly adsorbed, however, significantly increases the rate.