The rate of CO insertion into surface CHx species was investigated on silica-supported rhodium and rhodium-vanadium catalysts. Isotopically labelled 13CO was used in a transient kinetic experiment under steady-state conditions. A main conclusion is that vanadium promotion does not affect the rate of CO insertion. From temperature-programmed surface reaction measurements the surface concentration of CHx species during steady-state reaction was determined. It is increased by vanadium coadsorption. Combining these data with the selectivity and activity in synthesis gas to oxygenate conversion, a model is derived from which reaction rates of elementary steps and surface concentrations are calculated. Ethanal formation appears to occur at two distinguishable sites. Vanadium promotion decreases the desorption rate of ethanal, enhances the hydrogenation rate to ethanol, and increases the surface concentration of oxygenated intermediates.