A formalism for calculating spin-orbit coupling effects in organic molecules based on Rumer spin eigenfunctions and the second quantization approach has been derived by extending the method of Golebiewski and Broclawik [Int. J. Quantum Chem. 1985, 27, 613] to include one-electron spin-dependent operators. This formalism has been implemented for the semiempirical MNDOC-CI model and allows for a straightforward determination of potential energy as well as spin-orbit coupling surfaces within the context of configuration interaction calculations. Results for the magnitude and the geometry dependence of spin-orbit coupling in carbene, ethylene, and trimethylene are in good accord with ab initio data from the literature. The combined analysis of the potential energy and spin-orbit coupling surfaces in the case of trimethylene clearly demonstrates the power of this procedure for determining the triplet state reactivity.