The enthalpy of isomerization of peroxynitrite [oxoperoxonitrate (1-)] to nitrate (Delta H degrees(297), O=NOO- --> NO3-) was measured by solution calorimetry. Oxoperoxonitrate (1-) synthesized from both hydroxylamine and azide yielded enthalpies of isomerization of - 163 +/- 4 kJ mol(-1) at 24 degrees C. Since Delta(f)H(NO)(3)(-)) = - 205 kJ mol(-1), Delta(f)H(O=NOO-)= - 42 +/- 4 kJ mol(-1).