Herein, we have compared the behavior of ZnO and T in the photocatalytic oxidation of glycerol through detection of the main products. ZnO showed higher selectivity for the products where the carbon chain was preserved, such as glyceraldehyde (GAD) and dihydroxyacetone (DHA). TiO2 generated more products from the cleavage of the glycerol molecule, such as formaldehyde (FORM) and glycolaldehyde (GCOL). ZnO yielded about 16 times more GAD and 2.5 times more DHA than did TiO2.