Coupled cluster calculations through noniterative triple excitations were used to compute optimized structures, harmonic vibrational frequencies, atomization energies at 0 K, and heats of formation at 298 K for hydroxylamine (NH2OH) and three related compounds (NH3, HNO, and H2O2). The use of basis sets as large as augmented sextuple-zeta resulted in small extrapolations to the complete basis set limit in order to achieve chemical accuracy (+/-1 kcal/mol) in the thermodynamic properties. Complete basis set estimates were determined from several simple extrapolation formulas. In addition, four other corrections were applied to the frozen core atomization energies: (1) a zero-point vibrational correction, (2) a core/valence correlation correction, (3) a Douglas-Kroll-Hess scalar relativistic correction, and (4) a first-order atomic spin-orbit correction. For NH3 and HNO, we incorporated a fifth correction term intended to approximate the difference between coupled cluster theory and the full configuration interact result. This correction was based on coupled cluster theory through iterative quadruple excitations (CCSDTQ). Excellent agreement with experiment was found for the heats of formation of NH3, HNO, and H2O2. For NH2OH, the best current estimate of the heat of formation at 298 K is -10.1 +/- 0.3 kcal/mol, which falls roughly midway between two experimental values at -12.0 +/- 2.4 and -7.9 +/- 1.5 kcal/mol.