The spin-forbidden reaction 1HNO(1A’)+OH-→3NO-(3Σ-)+H2O has been extensively explored using various CASSCF active spaces with MP2 corrections in several basis sets. Natural bond orbital (NBO) analysis, together with the NBO energetic (deletion) analysis, indicates that the two isomers have nearly equal total energy and could compete with each other in the title reaction. More significantly, the singlet/triplet surface crossing regions have been examined and the spinorbit coupling (SOC) and energetics have been computed. The computational results indicate that the SOC is very large at the crossing point T1/S0 trans (ca.40.9 cm-1). Moreover, the T1/S0 trans has a low energy of 10.67 kcal/mol relative to that of trans-S0. Therefore, the surface crossing to the triplet state seems much more efficient at the T1/S0 trans region along the minimum energy path (MEP), However, The values of single (P1ISC) and double (P2ISC) passes estimated at T1/S0 trans show that the ISC occurs with a little probability. The spin-forbidden reaction 1HNO (1A ’) + OH- → 3NO- (3Σ -) + H2O has been extensively explored using various CASSCF active spaces with MP2 corrections in several basis sets. Natural Bond orbital (NBO) analysis, together with the NBO energetic (deletion) analysis, indicates that the two isomers have nearly equal total energy and could compete with each other in the title reaction. More significantly, the singlet / triplet surface crossing regions have been examined and the spinorbit coupling (SOC) and energetics The computational results indicate that the SOC is very large at the crossing point T1 / S0 trans (ca. 40.9 cm -1). Moreover, the T1 / S0 trans has a low energy of 10.67 kcal / mol relative to that of trans-S0. Thus, the surface crossing to the triplet state seems much more efficient at the T1 / S0 trans region along the minimum energy path (MEP), However, The values ​​of single (P1ISC) and double (P2ISC) at T1 / S0 trans show that the ISC occurs with a littl e probability.