Previous temperature-programmed desorption (TPD) and molecular beam studies have concluded that formaldehyde desorption is the only significant pathway in methanol oxidation at Cu(110) surfaces. We present here new TPD data for the adsorption of deuterated methanol (CD3OH) at a preoxidized Cu(110) surface and show that formate can be the major product of the reaction but that the experimental conditions adopted in earlier studies (specifically, preadsorption of oxygen and the adsorption of methanol at low temperatures) preclude formate formation. We propose that the reaction pathway is sensitive to the local structure of the methoxy/oxygen adlayer and suggest a model based upon this idea that reconciles the available experimental evidence. We also confirm the presence of two states of methoxy at the Cu(110)/O(a) surface and assign them to methoxy species with and without stabilization by surface oxygen.