Computational quantum theory is employed to determine the thermochemical properties of n-alkyl nitro and nitrite compounds: methyl and ethyl nitrites, CH3ONO and C2H5ONO, plus nitromethane and nitroethane, CH3NO2 and C2H5NO2, at 298.15 K using multilevel G3, CBS-QB3, and CBS-APNO composite methods employing both atomization and isodesmic reaction analysis. Structures and enthalpies of the corresponding aci-tautomers are also determined. The enthalpies of formation for the most stable conformers of methyl and ethyl nitrites at 298 K are determined to be -15.64 +/- 0.10 kcal mol(-1) (-65.44 +/-0.42 kJ mol(-1)) and -23.58 +/- 0.12 kcal mol(-1) (-98.32 +/- 0.58 kJ mol(-1)), respectively. Delta H-f degrees(298 K) of nitroalkanes are correspondingly evaluated at -17.67 +/- 0.27 kcal mol(-1) (-74.1 +/-1.12 kJ mol(-1)) and -25.06 +/- 0.07 kcal mol(-1) (-121.2 +/- 0.29 kJ mol(-1)) for CH3NO2 and C2H5NO2. Enthalpies of formation for the aci-tautomers are calculated as -3.45 +/- 0.44 kcal mol(-1) (-14.43 +/-0.11 kJ mol(-1)) for aci-nitromethane and -14.25 +/- 0.44 kcal mol(-1) (-59.95 +/- 1.84 kJ mol(-1)) for the aci-nitroethane isomers, respectively. Data are evaluated against experimental and computational values in the literature with recommendations. A set of thermal correction parameters to atomic (H, C, N, O) enthalpies at 0 K is developed, to enable a direct calculation of species enthalpy of formation at 298.15 K, using atomization reaction and computation outputs.