Global potential energy surfaces (PESs) of the [H,C,N,O] system in singlet and triplet states have been investigated using the hybrid density functional B3LYP/6-311G(d,p) method. Isocyanic acid, HNCO 1, has been found to be the most stable isomer for both multiplicities. The adiabatic singlet-triplet splitting for 1 is 82.6 kcal/mol. In the singlet state, HNCO is energetically followed by cyanic acid, HOCN 2, 28.7 kcal/mol higher than 1, fulminic acid, HCNO 3 (67.9 kcal/mol), and isofulminic acid, HONG 4 (87.1 kcal/mol). In the triplet state, the branched NC(H)O isomer 37 is 0.3 kcaymol higher than 31, followed by HOCN 32 (27.9 kcaMmol relative to triplet HNCO) and HCNO 33 (40.6 kcal/mol). The barriers for intramolecular rearrangements within singlet and triplet [H,C,N,O] system have been calculated to be high, and the isomerization processes in most cases are not expected to compete with fragmentations. Several minima on the singlet-triplet seam of crossing, relevant to the singlet [H,C,N,O] decomposition reactions, have also been found. The global features of the singlet and triplet PES have been applied to several important reactions, such as NH(3∑-)+CO, thermal decomposition of HNCO, O(3P)+HCN, O(3P)+HNC, and CH(2Π)+NO(2Π). For these reactions, major product channels have been speculated and their activation energies have been reported. Adiabatic ionization potentials for singlet and triplet [H,C,N,O] have been found to be high, in the range of 180-270 kcal/mol.