The structures and charge distributions of a number of bidentate bis(disubstituted-thiophosphorylamine) ligands used to extract zinc have been assessed by both semiempirical and ab initio molecular orbital methods and the results compared with crystallographic data where available. At the semiempirical level, the MNDO method gives superior results to either the AM1 or PM3 methods, which give poor charge distributions at the N, P, and S atoms. Five possible conformers of two tautomeric types were calculated for both alkyl and aryl derivatives. The results suggest that ligands containing an alpha-branched alkyl chain at phosphorus, which are good extractants for zinc, exist predominantly as cis SH tautomers with the acidic hydrogen covalently bonded to one sulfur and hydrogen bonded to the other. In contrast, those ligands containing a primary carbon or oxygen at phosphorus, which are poor extractants for zinc, are predicted to exist as NH tautomers with both sulfur atoms forming a trans arrangement to one another. These structural differences appear to explain the marked difference in the P-31 chemical shifts between the alpha-branched compounds and the other types.