The activation of CO 2 in thin potassium layers adsorbed on Cu(111) has been studied with time-evolved Fourier transform-infrared reflection absorption spectroscopy. The growth of thin layers by reactive evaporation of potassium in a CO 2 atmosphere permits control of the K:CO 2 stoichiometry, which strongly affects the selectivity in the formation of intermediates and the decomposition pathways of the layer. Layers grown in a CO 2 rich atmosphere exhibit the preferential growth of stoichiometric potassium oxalate K 2 C 2 O 4 (D 2 h ). The molecular identity of oxalate with D 2 h symmetry is confirmed by vibrational spectra utilizing isotopic substitution methods ( 1 3 CO 2 and C 1 8 O 2 ) and by the use of isotopic mixtures of CO 2 /C 1 8 O 2 and CO 2 / 1 3 CO 2 . A comparison of the isotope data with theoretical calculations gives an estimated OCO bond angle in oxalate of 132 o . Far-IR spectra obtained with synchrotron radiation indicate the equivalent interaction of all oxygen atoms with the potassium. A comparison of the vibrational data with theoretical ab initio calculations confirms the structural model of an oxalate species that is bulk coordinated with no strong directional bonding and all oxygen atoms equally interacting with potassium.At medium and low CO 2 :K ratios, very complex vibrational spectra are observed, indicating the formation of an oxalate surface species with C 2 v symmetry in addition to D 2 h - oxalate, CO 2 - and CO 2 2 - species.