The effective nuclear charges, which are empirical parameters in the approximate spin-orbit Hamiltonian, are determined for the first- and second-row elements in the periodic table using the experimental results of the fine structure splittings in the doublet and triplet II states of AH molecules (A is an atom in the first or second row). All calculations are performed using the full optimized reaction space multiconfiguration self-consistent-field wave functions with the 6-3 lG(d,p) basis set. Using these effective nuclear charges, the fine structure splittings calculated for the doublet and triplet II states of many diatomic molecules are in excellent agreement with experimental results. These charges are also applied to evaluate spin-orbit coupling constants connecting singlet and triplet states in some diatomic molecules and H2C0.