The formation, stability, adsorption geometry and electronic structure of a surface carbonate on Ni(100) have been investigated by photoemission (XPS, UPS) and temperature-programmed reaction (TPR). The core level binding energies of 531.2 eV for 0(1s) and 289.0 eV for C(1s) are comparable to those of bulk carbonates. The He(II) spectrum of the carbonate valence levels is not well defined because of the coexisting adsorbed and oxidic oxygen. The angular dependence of the carbonate core level intensities is characteristic of the carbonate being present as an overlayer species rather than a thicker surface phase. The XPS data and isotope labelled TPR experiments indicate the oxygen atoms of the carbonate to be electronically and chemically equivalent, and on this basis we favor a structure in which the carbonate is attached to the metal via all three oxygen atoms. This is supported by comparision with the core level binding energies of HCOOab and chemisorbed CO2,ad, which are similarly attached to the surface. From the core level angular behavior, the close similarity of core level binding energies and available vibrational spectroscopic data, a (nearly) planar geometry of the CO3,ad on Ni(100) is concluded, which is comparable to the planar bulk carbonate anion and the planar carbonate species on Ag(110). The activation barrier for decomposition is estimated from the observed maximum in TPR at 420 K to be 25 ± 2 kcal/mol. CO2 does not accumulate on the clean or Oad-precovered Ni(100) surface at 130 K. The stabilized, chemisorbed CO2,ad species often observed on other metal surfaces therefore does not play a critical role for carbonate formation on Ni(100). Also a mechanism involving the disproportionation of a CO2... CO2,ad- dimer anion can be ruled out from TPR data. The evidence of the experiments discussed in this paper suggests that the carbonate is predominantly formed by reaction of CO2,ad with a less stable, defect (disordered) Oad species rather than with isolated oxygen adatoms or with ordered Oad in the major ordered phases of Oad or NiO present on the Ni(100) surface. A comparison is made to existing literature on the CO2/O2 interaction and carbonate formation on other transition metal and Ag surfaces. The different reactivity for formation of carbonate on Ni as compared with Ag surfaces is explained in terms of an activated reaction, where the activation barrier is determined mainly by the energetic position of the initial state of Oad and CO2,ad and where the much higher adsorption energy of oxygen on nickel is mainly responsible for the much lower reactivity on that surface. © 1991.