The standard enthalpies of formation (DeltaH(f)(degrees) (298)) of a set of 16 biologically relevant reactive nitrogen oxide species were calculated using CBS-Q, CBS-QB3, CBS-APNO, G3 and G3X levels of theory. The G3 method gives the smallest mean absolute deviation of 5.1 kJ mol(-1), compared with available experimental (DeltaH(f)(degrees) (298)) values. Enthalpies of three reactions related to decomposition of ONOO- and ONOOH were also examined with various levels of theory. Large discrepancies are found for thermochemical results involving ONOO- using the CBS-APNO and CBS-RAD methods. This is due to the use of inadequate geometry predicted by the QCISD theory. (C) 2005 Elsevier B.V. All rights reserved.,