Irradiation of the disodium salt of the di-ptosylhydrazone of tetramethylcyclobutane-l, 3-dione in methanol generates 2, 2, 4-trimethylpent-3-enal dimethyl acetal and 2, 2, 4-trirnethylpent- knal, accompanied by methyl 2, 2, 4-trimethylpent-3-enoate, which waa subsequently found to be due to an impurity of the mono-ptosylhydrazone of tetramethylcyclobutane-l, 3-dione in the starting material. The mechanism of decomposition is discussed.