Metal coordination and isomerization of a hydrazone-based molecular switch (1) was investigated with isothermal titration calorimetry (ITC) and UV/Vis spectroscopy. Three equilibria—initial binding, ligand redistribution, and isomerization (1→1′)—are required to characterize the interaction of metal ions with the molecular switch. The stoichiometry of the initial complex and the relative magnitudes of the three equilibria constants depend on the size and Lewis acidity of the metal ion and competition from the counterion. Strongly coordinating anions (e.g., Cl−) suppress the formation of the initial metal–switch complex and isomerization. The thermodynamics for the equilibrium M2++2 1⇌M(1′)22+ have been determined for Zn2+, Cd2+, and Hg2+ from the ITC data.