The conversion of CO2 to CO via the electrolysis of molten Li2CO3 at 900 degrees C has a number of advantages including close to 100% faradaic and thermodynamic efficiencies, the latter limited by heat loss. Here we report that when the incoming gas contains SO2, sulfur uptake by the melt occurs only in the presence of oxygen and leads to the formation of Li2SO4. The electrolysis of molten Li2CO3/Li2O containing <2 mol% Li2SO4 produces CO, O-2 and elemental sulfur. Under these conditions, the decomposition potential of Li2SO4 is measured to be only 0.15 V, which is much lower than the decomposition potential of Li2CO3 (0.87 V). At sufficiently high cathode current densities (>1 A/cm(2)), the sulfur reduction current is diffusion limited and a steady state can be reached in which the amount of sulfur entering the melt equals the amount of sulfur leaving the melt. Thus even with SO2-containing sources of CO2, electrochemical conversion of CO2 to CO is able to proceed, but with reduced faradaic efficiency. Our findings suggest that the molten carbonate method for converting CO2 to CO can use flue gas from power stations as an inexpensive and readily available source of CO2. (C) 2013 The Electrochemical Society.