The photolysis of ethoxalyl azide gives ethoxalylnitrene and ethoxycarbonyl isocyanate. The nitrene adds to C=C double bonds and is inserted into O–H and C–H bonds. At high ethanol concentrations, where the singlet nitrene predominates, the O–H insertion product is formed dominantly. At low ethanol concentrations, where much of the nitrene changes to the triplet state, the hydrogen-abstraction product