The four catatopic complexes of general form Δ, Λ-[Ru(diimine)2(aa)]ClO4·nH2O (where diimine is bipy or phen and aa is S-threonine or S-allothreonine) have been isolated and each complex resolved into its two diastereoisomeric forms. Each isomer is photo-labile, equilibrating to a definite Λ|Δ ratio on light irradiation, with the resultant equilibrium constants reflecting chiral discrimination energies between isomeric pairs. Equilibration of these species have been followed both by circular dichroism (in H2O) and 1H NMR (in D2O) techniques. The NMR spectra have been fully interpreted for each of the eight isomeric forms. The S-threonine complexes equilibrate to Λ|Δ ratios of 0.77(2) H2O and 0.89(1) D2O for the phen isomers, and 0.68(1) H2O and 0.78(1) D2O for bipy. The corresponding values for the S-allothreonine complexes are 1.15(2) H2O, 1.09(1) D2O and 1.12(1) H2O, 1.02(2) D2O respectively. The crystal and molecular structures of the bipy complexes for both S-threonine and S-allothreonine have been determined. The two complexes are grossly isostructural. Δ, Λ-[Ru(bipy)2 (S-thr)]ClO4·5H2O is monoclinic, space group P21 with a = 10.165(2), b = 28.220(6), c = 10.896(3) Å, β = 104.80(2)° with Z = 4. Δ, Λ-[Ru(bipy)2 (S-allothr)] ClO4·5H2O is monoclinic, space group P21 with a = 10.226(4), b = 27.979(4), c = 10.875(2) Å, β = 104.99(2)° with Z = 4. Both structures were refined by block-matrix least- squares methods to R 0.042 and 0.037 for 3327 and 4456 non-zero reflexions respectively. A pseudo a-glide operation relates the two Δ and Λ forms in the asymmetric unit of each structure. There are no apparent significant differences between corresponding parameters in the four cations. The average bond lengths are RuO 2.094(9). RuNamine 2.128(11) and Rubipy 2.050(20) Å with average bite angles at Ru for the amino acids of 79.7(5) and for bipy ligands of 79.0(4)°. The amino acid chelate rings in the four molecular structures have like conformations, suggesting only minor differences in steric factors within these cations. There is evidence in the crystal structures that in the S-allothr complexes an internal H-bond may exist between the β-OH and carboxylic groups which would be unfavourable in the S-thr complexes. This structural difference in the two amino acid side chains is consistent both with observed differences in the NMR spectra of the complexes, and with the different equilibration ratios obtained. Close linear contacts between the hydroxy oxygen atoms and the 3,3′-protons in the crystals suggest that the latter may have weak carbon acid character.