The influence of deuteration on the adsorption and oxidation of ethylene glycol (EG) and deuterated EG (HOCD2CD2OH, "EG-D-4") on a Pt thin film electrode was investigated in a combined spectro-electrochemical approach under continuous flow conditions, aiming at an improved understanding of the reaction mechanism. Highly surface sensitive in situ ATR-FTIR spectroscopy was employed to follow the potential dependent development of the EG adlayer, while online differential electrochemical mass spectrometry (DEMS) was used to monitor the volatile products simultaneously. Potentiodynamic, potentiostatic and adsorbate-stripping experiments show distinct kinetic isotope effects (KIEs), which differ significantly for different reaction pathways such as the formation of CO2, COad and of the precursor for COad formation, and finally the overall reactivity expressed by the Faradaic current. The data indicate that the precursor for COad formation, identified by a band at 1630 cm(-1), includes at least one H ( or D) atom at the second C atom, pointing to an adsorbed 2-hydroxyacetyl, and that C-C bond dissociation is most likely not the rate determining step for COad formation. Implications of these findings on the mechanism of EG adsorption/oxidation are discussed. (C) 2012 Elsevier B.V. All rights reserved.