The size extensive, ab initio effective valence shell Hamiltonian method, which is based on quasidegenerate many-body perturbation theory, has been extended to treat spin-orbit coupling in atoms or molecules. Just as the exact projection of the nonrelativistic Hamiltonian into a prechosen valence space enables deriving the multireference perturbation expansion for the exact effective valence shell Hamiltonian, the addition of the Breit–Pauli spin-orbit operator to the original Hamiltonian (as an extra perturbation) enables the use of quasidegenerate many-body perturbation theory to produce the energy independent effective spin-orbit coupling operator that acts within the prechosen valence space. To assess the accuracy of the proposed method, test calculations are performed for the spin-orbit splittings in the valence states of C, Si, Ge, CH, SiH, and GeH and their positive ions using the one-electron spin-orbit approximation with standard values of the effective nuclear charge. The computed spin-orbit spl...