Abstract Intermediate states, following the exposure of bulk polycrystalline potassium to carbon dioxide, were identified by comparisons between spectroscopic information and calculated data. We utilize three routes for the identification of each intermediate: (i) vibrational spectra, (ii) ionization potentials, and (iii) total energies. Molecular CO2 adsorbs at low temperatures with limited site specificity. CO2 molecules also form oxalate ions, which penetrate into the potassium overlayer. These ‘mineral oxalates’ decompose to bulk coordinated carbonates. Our calculated data also allow for the identification of CO2 derivatives on surfaces promoted by thin overlayers of alkali metals.