The mechanism of methanol dehydrogenation to methyl formate (MF) was studied over a copper-zirconium alloy catalyst sample prepared from an amorphous precursor. As a result of a special activation procedure, a catalyst of stable activity was obtained suitable for mechanistic studies. A mixture of CH3OD and CD3OD was used in various ratios to study the isotope distribution of unreacted methanol and product MF. The D/H ratio in the formyl group is similar to that of the feed mixture. Similarly, considerable D/H randomization was found in the methyl group of methyl formate. These results suggest that fast dehydrogenation-hydrogenation equilibrium steps occur prior to the formation/desorption of methyl formate. The mechanism of the reaction can be rationalized assuming an intermediate which we call equilibrated surface formyl/formate group.