3,6‐Dihydro‐1,3,4‐oxadiazin‐2‐ones may be prepared from either α‐ketols or from α‐diketones and carbethoxyhydrazine. These substances undergo thermal or photolytic (253.7 nm) decomposition to give N2, CO2 and olefins. Oxadiazinones derived from medium ring α‐ketols give bicyclic hydrocarbons in addition to olefins. These are shown to be formed by decomposition of the intermediate vinyldiazene to vinyl radicals, followed by transannular hydrogen abstraction and alkyl radical addition to the olefin. A series of 5,6‐dihydro‐1,3,4‐oxadiazin‐2‐ones have also been prepared and their thermal and photolytic behavior studied.