Electrochemical fluorination on a nickel anode (the Simons process) is an important industrial routetowards fluorinated compounds. Despite its success, the mechanism is under debate. In the presentpaper we report, for the first time, a first-principles study of adsorption of CO on a twice-oxidisedNiF 2(F 2) (001) surface, a model surface representing the possible catalytically active surface of theanode. Adsorption of only the CO results primarily in the formation of COF 2 , which is a thermody-namically preferred outcome. Further, to account for the liquid hydrogen fluoride environment weco-adsorbed up to two HF molecules. This gave rise to a variety of different adsorption outcomeson the surface which we grouped together depending on their structure. Alongside the formationof COF 2 as a main adsorption product, also alcohol F3 COH and F2 HCOF can be formed. If sufficientHF is supplied even CO can be split and CF4 and water are possible reaction outcomes. Furthermore,also the two co-adsorbed HF molecules can split and form an F2 molecule.