Kinetics and mechanisms for the reactions of HNO with CH3 and C6H5 have been investigated by ab initio molecular orbital (MO) and transition-state theory (TST) and/or Rice-Ramsperger-Kassel-Marcus/ Master Equation (RRKM/ME) calculations, The G2M(RCC, MP2)//B3LYP/6-31G(d) method was employed to evaluate the energetics for construction of their potential energy surfaces and prediction of reaction rate constants. The reactions R + HNO (R = CH3 and C6H5) were found to proceed by two key product channels giving ( 1 ) RH + NO and (2) RNO + H, primarily by direct abstraction and indirect association/decomposition mechanisms, respectively. As both reactions initially occur barrierlessly, their rate constants were evaluated with a canonical variational approach in our TST and RRKM/ME calculations. For practical applications, the rate constants evaluated for the atmospheric-pressure condition are represented by modified Arrhenius equations in units of cm3 mol-1 s-1 for the temperature range 298-2500 K: script k sign 1A = 1.47× 10 11T0.76exp[-175/T], script k sign2A = 8.06 × 103T2.40exp[-3100/T], script k sign1B = 3.78 × 105T2.28exp[230/T], and script k sign 2B = 3.79 × 109T1.19exp[-4800/T], where A and B represent CH3 and C6H5 reactions, respectively. Based on the predicted rate constant at 1 atm pressure for R + HNO → RNO + H, we estimated their reverse rate constants for R + HNO production from H + RNO in units of cm3 mol-1 s -1 script k sign-2A′ = 7.01 × 10 10T0.84exp[120/T and script k sign2B′ = 2.22×1019T-1.01exp[-9700/T].The heats of formation at 0 K for CH3NO, CH3N(H)O, CH3NOH, C 6H5N(H)O, and C6H5NOH have been estimated to be 18.6, 18.1, 22.5, 47.2, and 50.7 kcal mol-1 with an estimated ≠1 kcal mol-1 error.