In this work we investigate the electro-oxidation of glycerol reaction on polycrystalline platinum in acidic media. By using in situ FTIR and isotopically labeled glycerol ( 13 CH 2 OH– 12 CHOH– 13 CH 2 OH) we demonstrate the co-existence of 13 CO 2 and 12 CO 2 as products of electro-oxidation. Results indicate that glycerol completely dissociates on Pt surfaces and that both groups contribute to the production of CO 2 . Moreover, terminal 13 CH 2 OH groups are easier to electro-oxidize than the central group, which is interpreted in terms of a more favorable position of – 13 CH 2 OH groups to react with Pt–OH ad species.