The NCN diradical is an important intermediate of prompt nitric oxide formation in flames. The mechanism of intersystem crossing (ISC) in the NCN molecule formed via pyrolysis or photolysis of NCN3 is of relevance to the interpretation of experiments that utilize NCN3 as a precursor for laboratory studies of NCN kinetics. This mechanism has been investigated by means of multi-reference configuration interaction calculations. From the potential energy surfaces for NCN3 dissociation, it was inferred that both thermal and photo-chemical decomposition initially lead to NCN in its lowest singlet state, ã1Δg, with a possible contribution from the b̃1Σg+ state at low photolysis wavelengths. Direct formation of the triplet ground state X̃ 3Σg- is also feasible for the photolytic pathway. An analysis of surface crossings between ã or b̃ and the triplet ground state X̃ 3Σg- in the absence and presence of a helium atom revealed an ISC channel NCN1(ã)→3NCN(X̃) via a strongly bent structure. However, its barrier of 38 kcal mol-1 relative to the singlet minimum turned out to be much too high to explain the fast ISC observed in experiments. A rigid-bender model including Renner-Teller interactions was used to examine the occurrence of mixed-multiplicity rovibrational states-so-called gateway states-that could enhance collision-induced ISC. The results of this study indicate that a gateway mechanism is probably not operative in the case of the ã/X̃ pair of states in NCN.