2H-1,4-Benzoxazin-3(4H)-one, a benzoxazine derivative, is a heterocyclic building block for various natural and synthetic organic compounds. It has been reported as an intermediate during the biogenesis of cyclic hydoxamic acids in maize. Its standard molar enthalpy of formation and tautomerization energy of its tautomers has been evaluated by calorimetric and computational methods. It has been synthesized by reacting o-aminophenol with chloroacetyl chloride in the presence of butanone and aqueous NaHCO3.