A critical review of the heat of formation of HNO is presented. This molecule and its thermodynamics parameters play crucial roles in the chemical mechanisms of propellant combustion and NOx pollutant chemistry. It was found that predissociation experiments, which have gone largely unnoticed for about 15 years, lead to a significant revision in the recommended value. The new value, 25.6(-0.1)(+0.6) kcal/mol (298 K; 26.3 kcal/mol at 0 K), is 1 to 2 kcal/mol higher than previous recommendations and has much narrower error limits. Heats of formation of NO, NH2OH, HNO+, and HNO- are also briefly examined, and recommendations are made.