The reaction between CO2 and C2H4 in the presence of H-2 over Pt and PtSn-based (SiO2, Al2O3, MgO) supported catalysts has been studied at 34 bar of total pressure and 423 K by in situ diffuse reflectance infrared Fourier transform spectroscopy and on-line mass spectrometry experiments. Over silica-supported PtSn alloy, the reaction yields 2-hydroxypropanoic acid through a C2-carbonylic species and CO2 nondissociatively coordinated. C2H4 chemisorption on PtSn/SiO2 produces a C2-carbonylic species which interacts with the hydroxyl groups of SiO2, and this C2-carbonylic species evolves to acetaldehyde at 423 K. 2-Hydroxypropanoic acid is also produced over PtSn/SiO2 from acetaldehyde, CO2, and H-2, but not from ethanol and CO2. (C) 2001 Academic Press.