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Metallothioneins (MTs) are a ubiquitous class of small metal-binding proteins involved in metal homeostasis and detoxification. While known for their high affinity for d 10 metal ions, there is a surprising dearth of thermodynamic data on metals binding to MTs. In this study, Zn 2+ and Cu + binding to mammalian metallothionein-3 (MT-3) were quantified at pH 7.4 by isothermal titration calorimetry (ITC). Zn 2+ binding was measured by chelation titrations of Zn 7 MT-3, while Cu + binding was measured by Zn 2+ displacement from Zn 7 MT-3 with competition from glutathione (GSH). Titrations in multiple buffers enabled a detailed analysis that yielded condition-independent values for the association constant ( K ) and the change in enthalpy (Δ H ) and entropy (Δ S ) for these metal ions binding to MT-3. Zn 2+ was also chelated from the individual α and β domains of MT-3 to quantify the thermodynamics of inter-domain interactions in metal binding. Comparative titrations of Zn 7 MT-2 with Cu + revealed that both MT isoforms have similar Cu + affinities and binding thermodynamics, indicating that Δ H and Δ S are determined primarily by the conserved Cys residues. Inductively coupled plasma mass spectrometry (ICP-MS) analysis and low temperature luminescence measurements of Cu-replete samples showed that both proteins form two Cu 4 + –thiolate clusters when Cu + displaces Zn 2+ under physiological conditions. Comparison of the Zn 2+ and Cu + binding thermodynamics reveal that enthalpically-favoured Cu + , which forms Cu 4 + –thiolate clusters, displaces the entropically-favoured Zn 2+ . These results provide a detailed thermodynamic analysis of d 10 metal binding to these thiolate-rich proteins and quantitative support for, as well as molecular insight into, the role that MT-3 plays in the neuronal chemistry of copper.