Tris(2-mercaptobenzyl)amine, S3N, and tris(2-hydroxybenzyl)amine, O3N, were investigated with In3+ and Ga3+ in solution and in the solid state to help interpret the contrasting in vivo behavior of their complexes as diagnostic imaging probes. To this end the protonation and metal binding constants were determined and their similarities and differences with those of analogous ligands, including the O3N analogue, are discussed. Seven new crystal structures of Ga(III) and In(III) complexes are also reported : [Ga(S3N)], [Ga(S3N)(1-Me-imid)], [In(S3N)(1-Me-imid)], [In(S3N)(DMF)], [Ga(O3N)(1-Me-imid)], [Ga(O3N)(DMF)], and [In(O3N')(1-Me-imid)](2), where O3N' is the tris(2-hydroxy-3,5-dimethylbenzyl)amine homologue of O3N. The structure of [Ga(S3N)] is 4-coordinate, that of [In(O3N')(1-Me-imid)(2)] is 6-coordinate, and the remaining structures are 5-coordinate.