A series of double-chain surfactant octahedral Co(III) complexes 1-4 has been prepared in both water and 20:1 (v/v) ethanol-water by the reaction of an N-alkylethylenediamine ligand (5) with sodium hexanitrocobaltate(III), followed by the addition of sodium perchlorate or sodium nitrate : trans-dinitro-cisoid-bis(N-alkylethylenediamine)cobalt(III) perchlorate (1) and nitrate (3), and trans-dinitro-transoid-bis(N-alkylethylenediamine)cobalt(III) perchlorate (2) and nitrate (4) (alkyl = hexyl, octyl, decyl, dodecyl, and hexadecyl). The cisoid 1(3)/transoid 2(4) diastereomer ratio increased on going from 20:1 ethanol-water to water, in particular by a factor of approximately 5 for the octyl and dodecyl perchlorate systems and approximately 8 for the hexadecyl perchlorate system. The increase in the relative amount of cisoid 1(3) is attributed to the hydrophobic effect associated with the aggregation of ligand 5 in water.