Electrochemical properties of Mg-, Fe- or Co-substituted K2NiF4-type Sr2TiO4-based oxide solid solutions were investigated and compared to those of indium substituted system, which had been confirmed to be a protonic conductor [1]. All samples were prepared using the conventional solid-state reaction method. The solubility limit of Mg for Sr2TiO4 was about 0.05, just the half of solubility limit of In, 0.1. The dependence of electrical conductivities of Sr2Ti1-xMgxO4-delta on x was very similar to that of conductivities of Sr2Ti1-xInxO4-delta on x/2. EMF's of hydrogen or oxygen concentration cells using Sr2Ti0.97Mg0.03O4-delta as a solid electrolyte were the same as those using Sr2Ti0.93In0.07O4-delta. These results indicate that in the Sr2TiO4-based system the predominant factor governing the phase relation and protonic conduction properties is the concentration of introduced oxygen vacancies. Single phase samples of Fe- or Go-substituted solutions, Sr2Ti0.93Fe0.07O4-delta and Sr2Ti0.93Co0.07O4-delta were obtained. In Sr2Ti0.93Fe0.07O4-delta, the dependence of conductivity on oxygen partial pressure was qualitatively equal to that of Sr2Ti0.93In0.07O4-delta indicating that the valence state of Fe is settled at + 3 (3d(5)) in Sr2Ti0.93Fe0.07O4-delta and five d-electrons of Fe3+ are localized. In Sr2Ti0.93Co0.07O4-delta, the dependence of conductivity on oxygen partial pressure was different from those of Sr2Ti1-xMxO4-delta (M = In, Mg and Fe). There were neither pure ionic conduction regions at intermediate P-O2 nor n-type electronic contributions to conductivity in low P-O2 in Sr2Ti0.93Co0.07O4-delta.