The C-13 chemical shifts of methylcobalt(III) compounds with saturated amine ligands in cis positions to the methyl group and a monodentate ligand, L = CN-, NH3, NO2, N-3(-), H2O, or OH-, in the trans position are reported. The amine ligands used, 1,2-ethanediamine (en), 1,3-propanediamine (tn), N,N'-bis(2-aminoethyl)-1,3-propanediamine (2,3,2-tet), N,N'-bis(3-aminopropyl)-1,2-ethanediamine (3,2,3-tet), and 1,4,8,11-tetraazacyclotetradecane (cyclam), all exert an apparent cis influence on the 13 C resonance signal of the coordinated methyl group. In the trans-[Co-(en)(2)(CH3)(L)](n+) series the N-15 resonance frequency of the coordinated en has also been measured. The influence of L on the en N-15 chemical shifts is reverse the influence on the methyl C-13 chemical shifts except in the case of L = NO2-, which affects a further deshielding of the amine nitrogen nucleus. The methyl (1)J(CH) coupling constants in the trans-[Co(en)(2)(CH3)(L)](n+) series range from 128.09 Hz (L = CN-) to 134.11 Hz (L = H2O). The crystal structures of trans-[Co(en)(2)(CH3)(ClZnCl3)], trans-[Co(3,2,3-tet)(CH3)(N-3)]ClO4, trans, trans-[(CH3)(en)(2)CO(CN)Co(en)(2)(CH3)](PF6)(3)(CH3CN), and cis-[Co(en)(2)(CH3)(NH3)]ZnCl4 were determined from low-temperature X-ray diffraction data.