The diazoketone derived from tricyclo[4.1.0.0 1 , 3 ]heptan-4-one on photolysis in dimethylamine leads to a novel product that appears to be formed via a cyclopropylcarbene rearrangement followed by dimethylamine addition and the cleavage of one of the cyclobutane rings. In a matrix at 15K, photolysis led to the loss of the 2085 cm - 1 diazo band and the formation of a new band at 2117 cm - 1 which is characteristic of a small ring ketene such as cyclopropylketene.