The coordination chemistry of the mixed donor 12-membered macrocyclic ligand 1-aza-4,10-dithia-7-oxacyclododecane ([12]aneNS₂O) with Cu^II, Zn^II, Cd^II, Hg^II, and Pb^II has been investigated both in water solution and in the solid state. The protonation constant for [12]aneNS₂O and stability constants with the aforementioned metal ions have been determined potentiometrically and compared with those reported for other mixed N/S/O-donor tetradentate 12-membered macrocycles. The measured values are consistent with trends observed previously for aza macrocycles as secondary N-donors are replaced by O- and S-donors. In particular our results show that Hg^II in water has the highest affinity for [12]aneNS₂O followed by Cu^II, Cd^II, Pb^II, and Zn^II. For each considered metal ion, 1 ∶ 1 complexes of [12]aneNS₂O have been isolated in the solid state; those of Cu^II, Hg^II, and Cd^II have also been characterised by X-ray crystallography. In the cases of copper(II) and cadmium(II) complexes the ligand adopts a folded [2424] conformation, whereas a more planar [3333] conformation is observed in the case of the mercury(II) complex. The macrocycle [12]aneNS₂O and its structural analogue [12]aneNS₃ have then been used as receptor units in the design and synthesis of the new ferrocene-containing redox-active ionophores N-ferrocenylmethyl 1-aza-4,10-dithia-7-oxacyclododecane (L¹) and N-ferrocenylmethyl 1-aza-4,10,7-trithiacyclododecane (L²). Electrochemical studies carried out in MeCN in the presence of increasing amounts of Cu^II, Zn^II, Cd^II, Hg^II, and Pb^II showed that the wave corresponding to the Fc/Fc⁺ couple of the uncomplexed ionophores L¹ and L² is gradually replaced by a new reversible wave at more positive potentials and corresponding to the Fc/Fc⁺ couple of the complexed ionophores. The maximum shift of the ferrocene oxidation wave was found for L¹ in the presence of Zn^II (230 mV) and Pb^II (220 mV), whereas for L² a selective sensing response for Cu^II over the other guest metal cations was observed with an oxidation peak shift of 230 mV.