Direct and sensitized irradiation of methyl diazomalonate gives singlet and triplet biscarbomethoxycarbene, respectively. The reactions of these species with carbon–carbon double bonds are described. Cyclopropanes are formed with substantial retention of stereochemistry from the singlet but with complete loss from the triplet. Additions to aliene, norbornadiene, and benzene are also achieved. The reactions of several triplet carbenes with dienes are reported, but even in systems thought to be favorable to 1,4 addition, only the normal cyclopropanes could be found. Reactions with carbon-hydrogen single bonds are also described. Singlet carbene reacts by direct insertion, but the triplet abstracts hydrogen to give radical pairs which undergo recombination and disproportionation.