The reaction of [Cu(L)(H₂O)]²⁺ with an excess of thiosulfate in aqueous solution produces a blue to green color change indicative of thiosulfate coordination to Cu(II) [L = tren, Bz₃tren, Me₆tren, and Me₃tren; tren = tris(2-aminoethyl)amine, Bz₃tren = tris(2-benzylaminoethyl)amine, Me₆tren = tris(2,2-dimethylaminoethyl)amine, and Me₃tren = tris(2-methylaminoethyl)amine]. In excess thiosulfate, only [Cu(Me₆tren)(H₂O)]²⁺ promotes the oxidation of thiosulfate to polythionates. Products suitable for single-crystal X-ray diffraction analyses were obtained for three thiosulfate complexes, namely, [Cu(tren)(S₂O₃)]·H₂O, [Cu(Bz₃tren)(S₂O₃)]·MeOH, and (H₃Me₃tren)[Cu(Me₃tren)(S₂O₃)]₂(ClO₄)₃. Isolation of [Cu(Me₆tren)(S₂O₃)] was prevented by its reactivity. In each complex, the copper(II) center is found in a trigonal bipyramidal (TBP) geometry consisting of four amine nitrogen atoms, with the bridgehead nitrogen in an axial position and an S-bound thiosulfate in the other axial site. Each structure exhibits H bonding (involving the amine ligand, thiosulfate, and solvent molecule, if present), forming either 2D sheets or 1D chains. The structure of [Cu(Me₃tren)(MeCN)](ClO₄)₂ was also determined for comparison since no structures of mononuclear Cu(II)−Me₃tren complexes have been reported. The thiosulfate binding constant was determined spectrophotometrically for each Cu(II)−amine complex. Three complexes yielded the highest values reported to date [K_f = (1.82 ± 0.09) × 10³ M^-1 for tren, (4.30 ± 0.21) × 10⁴ M^-1 for Bz₃tren, and (2.13 ± 0.05) × 10³ M^-1 for Me₃tren], while for Me₆tren, the binding constant was much smaller (40 ± 10 M^-1).