The structures of the diastereoisomers Δ (+)_578-, (I), andΔ(-)_578--bis-(ethane-1,2-diamine)[β-ethyl (S)-aspartato-k² N,O¹]cobalt(III) bis-(perchlorate) monohydrate, (II), both [Co(C₆H₁₀N₂O ₄)(C₂H₈N₂)₂](ClO ₄)₂·H₂O, are compared. In both structures, the ester group of the amino acid side chain is engaged only in intra-molecular hydrogen bonding to coordinated amine groups. This inter-action is stronger in (I) and correlates with previously observed diastereoisomeric equilibrium ratios for related metal complex systems in aqueous media. The two perchlorate anions of (II) are located on twofold axes. Both perchlorates in (I) and one of the perchlorates in (II) are affected by disorder. Both structures exhibit extensive three-dimensional hydrogen-bonding net-works.