A wet chemical route based on constant-potential electrolysis and different reactivities of ${\mathrm{Cu}}^{3+}$ and ${\mathrm{Tl}}^{3+}$ with water is proposed to distinguish the average valences of Cu and Tl in superconducting thallium cuprates. The validity of the method is demonstrated using ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$, ${\mathrm{Tl}}_2$${\mathrm{O}}_3$, and ${\mathrm{Tl}}_2$O as reference materials for ${\mathrm{Cu}}^{3+}$, ${\mathrm{Tl}}^{3+}$, and ${\mathrm{Tl}}^{+}$, respectively. Furthermore, the oxygen contents and the cation valences have been determined for various Tl-22(n-1)n phases (where n=1–4). With increasing n value the Cu valence is found to decrease, while the valence of thallium remains rather constant at ca. +3, regardless of the value of n. Based on these results, the possible carrier doping mechanisms in the Tl-22(n-1)n phases are discussed.

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