Photocatalytic polyol conversion provides a green approach for the synthesis of value-added products. However, efficient and selective photocatalysts that can prevent unwanted full oxidation are still missing, mostly due to a lack of mechanistic understanding. Here we use ethylene glycol (EG) as model compound to study the reaction pathways in photocatalytic polyol dissociation under aerated conditions using in situ vibrational spectroscopy coupled with mass spectrometry. On pristine TiO2, the presence of oxygen leads to the formation of formaldehyde via photocatalytic CC bond cleavage, where the removal of photo-generated surface adsorbed proton (H-ads(+)) in the form of water is the rate determining step (RDS). The photo-generated formaldehyde molecules subsequently convert into CO2 via complete oxidation by oxygen, or into paraformaldehyde by polymerization with water. A promotion effect is observed when noble metal (Au, Pt, Ag) nanoparticles (NPs) are used as cocatalysts. While Ag and Au selectively promote the formation of paraformaldehyde, the addition of Pt facilitates the complete oxidation of EG into CO2. By performing the reaction under a low oxygen partial pressure, we rationalize that Ag and Au NPs accelerate the polymerization of formaldehyde by providing water rapidly through direct oxidation of H-ads oxidation, whereas Pt NPs supply water indirectly, in a pathway via H-2 or formaldehyde oxidation. (C) 2018 Elsevier Inc. All rights reserved.