The nitrous oxides released when ethylene oxide is introduced at 30.degree.-70.degree.C. into aqueous nitric acid of 50-70 percent by weight of HNO.sub.3 is recirculated thereto and gaseous oxygen is introduced into the nitric acid. A post-oxidation holding of the oxidation mixture for 8-24 hours at 30.degree.-70.degree.C. increases the yield of oxalic acid.
