Irradiation of 4-methyl-2-phenyl-1, 3-oxazin-6-one, 1, yields a photochemical equilibrium between 1 and 2-methyl-4-phenyl-1, 3-oxazin-6-one, 2. The isomerization proceeds through two intermediates, the oxa-azabicyclo [2.2. 0] hexenones 3 and 4, both of which can be observed spectroscopically (ir, uv, 1H nmr) at low temperature.