It is shown that the effect of low magnetic field on the net spin direction of an excited triplet sample at low temperatures depends on the zero-field parameters of the triplet state coupled to the lowest singlet state in the intersystem crossing process. Using this idea and double resonance techniques, the mechanism of the ISC in pyrazine, quinoxaline and benzophenone is determined. In nitrogen heterocyclics, the type of electronic excitation is changed in the ISC process (i.e., n, π* ↔ π, π* selection rules are followed). In benzophenone, a direct mechanism between singlet and triplet states of the modified n, π* type in the nonplanar configuration is involved. The effect of nonplanarity on the spin orbit interaction in heterocyclics is discussed in detail.