Dimethyl 3,6-oxy-3,4,5,6-tetrahydrophthalate was hydrolysed with aqueous sodium hydroxide and the corresponding 3,6-oxy-3,4,5,6-tetrahy drophthalic acid was obtained in 96% yield. Liberation of ethylene from this acid by heating with quinoline at 170C gave furan-3,4, dicarboxylic acid in 85% yield.