The effect of oxygen on the activity and selectivity of a Pd-Fe/alpha-Al2O3 catalyst for CO coupling to diethyl oxalate (DEO) was studied in the range of 105 - 120degreesC. The results showed that the conversion of CO and the STY (Space Time Yield) of DEO increased with increasing oxygen concentration from 5 to 10 mol% and reaction temperature, but the selectivity to DEO decreased. The role of oxygen in CO coupling is discussed on the basis of the experimental results in the present work and the XPS characterization of the catalysts. It was also found that the activity and selectivity of the catalyst could be fully recovered after removing O-2 from the reaction system.