Linear and nonlinear halogen dependencies of the 13 C magnetic shielding constants of CH 4−n I n , CH 4−n Br n , CCl 4−n I n , and CBr 4−n I n were fairly reproduced by the ab initio generalized unrestricted Hartree-Fock (GUHF)/finite perturbation (FP) method including spin-orbit (SO) interaction and spin-free relativistic (SFR) terms. As seen from the experimental trends, the calculated 13 C chemical shifts in CCl 4−n I n and CBr 4−n I n depend linearly on n = 0-4, while those in CH 4−n I n and CH 4−n Br n depend nonlinearly. We found that both the linear and nonlinear dependencies are due to the relativistic effects, and especially due to the Fermi-Contact (FC) term originating from the SO interaction.