Abstract The electromotive force (emf) of a solid-state galvanic cell, Ag|AgI|PT–In, where PT–In is the phenothiazine-iodine complex, was examined as functions of the composition, n, and also of the temperature. Below n=2.5, a value of 616 mV was obtained at 25 °C. The emf increases stepwise to 620 mV at n=2.5 in conformity with the formation of a complex cation radical pentaiodide, as established by our earlier work. A continuous increase up to 649 mV was found before n=3, indicating the appearance of a nonstoichiometric phase covering the range from n=2.8 to 3. The upper limit of the existence region exactly corresponds with the minimum in the resistivity-composition isotherm. Similar studies with benzo[a]- and benzo[c]phenothiazines revealed the formation of three or four distinct nonstoichiometric complexes in each system. The Gibbs energy, enthalpy, and entropy of cell reactions and complex formation reactions are presented. The stability of the iodine complexes was found in the following order: benzo[a]phenothiazine>benzo[c]phenothiazine>phenothiazine.