Oxygen → Tetrahydrophthalic anhydride

3Me--THPA was produced by the liquid phase aeration method. Thus, the air in a 300 ml four-necked flask similar to the one used in Example 5 was replaced with nitrogen and, then, the flask was charged with 98 g of maleic anhydride, 300 ppm of p-tert-butylcatechol by way of auxiliary polymerization inhibitor and 800 ppm of dilauryl thiodipropionate by way of auxiliary polymerization inhibitor and the charge was melted at 60° C. Then, the internal atmosphere of the reaction system was replaced with nitrogen gas, and thereafter nitrogen gas containing 1 volume % of oxygen was passed into the reaction mixture at a rate of 5 ml/min throughout the reaction. Under this atmosphere, 374 g of crude trans-1,3-pentadiene of the same composition as that used in Example 1 was continuously added at 60° C. for 4 hours. Major portion of unreactive fractions was distilled off from the reaction mixture in the course of reaction. Thereafter, the reaction mixture was treated in the same manner as in Example 5 to give 164 g of 3Me--THPA. The investigation revealed no gels in the product.