Single-crystal X-ray structural determinations have been recorded at c.295 K for the tris ( dithiocarbamato )cobalt(III) compounds [Co(S2CNMePh)3] and [Co(S2CNPh2)3]. [Co(S2CNMePh)3] crystallizes in space group P21/a with a 25.207(6), b 12.778(2), c 9.497(2)?, ߠ116.40(2)஠[Co(S2CNPh2)3] crystallizes in space group Pi with a 14.979(6), b 13.169(4), c 10.683(3)?, a 71.53(3),ߠ88.15(3),? 82.35(3)஠The structures were refined to residuals of 0.039 and 0.062 for 2184 and 4068 'observed' reflections respectively. The cobalt-59 n.m.r. chemical shift of the sparingly soluble diphenyl compound is found to be 6450 ppm downfield from K3Co(CN)6, compared with 6660 ppm for the methyl phenyl compound and 6830 ppm for the non-phenyl-substituted dimethyl compound, a result reflecting increase in the ligand -field strengths of the ligands in this series of compounds in the order Me2 < Me Ph < Ph2. Average geometric parameters (?) are: methyl phenyl, Co-S 2.270(6), C-S 1.699(6), C-N 1.341(9), S哠2.803(2), N-Cphenyl 1.446(5), N-Cmethyl 1.45(1); diphenyl, Co-S 2.267(8), C-S 1.699(4), C-N 1.335(4), S哠2.805(2), N-Cphenyl 1.449(6). Solid-state carbon-13 chemical shifts for thecarbon atom of the NCS2 fragment are 204, 207 and 210 ppm for [Co(S2CNMe2)3], [Co(S2CNMePh)3] and [Co(S2CNPh2)3] respectively. By comparison, the carbon-13 chemical shifts for cobalt complexes with saturated hydrocarbon substituents which span the range of ligand -field strengths do not show the same systematic trends: [Co(S2CN(CH2)4)3], 201 ppm ; [Co(S2CNEt2)3], 204 ppm ; and [Co(S2CNPri2)3], 204 ppm , [Co(S2CN(CH2Ph)2)3], 207 ppm . For the unsaturated hydrocarbon system, [Co(S2CN(CH)4)3], this value is 211 ppm . These results correspond closely to solution carbon-13 data and constitute good evidence that the ligand substituent effects in the compounds are similar in the two states. The increase in ligand -field strength with phenyl substituent is rationalized in terms of a combination of steric and electronic effects.