•Solid solution limit of the Sn1−x(Zn2/3Sb1/3)xP2O7 compounds to be x = 0.15.•Highest value of 2.2 × 10–2 S cm−1 is obtained for x = 0.15 at 250 °C.•Formation of oxygen vacancies is caused by cosubstitution of Zn and Sb for Sn.•Interstitial proton of lattice is introduced by formation of oxygen vacancies.•The proton mobility is enhanced by the cosubstitution of Zn and Sb for Sn. In order to investigate the effects of the Zn and Sb cosubstitution for Sn on the crystal structure and electrical conductivity of SnP2O7, Sn1−x(Zn2/3Sb1/3)xP2O7 compounds were synthesized in this study. The crystal structure refinement results show the solid solution limit of the Sn1−x(Zn2/3Sb1/3)xP2O7 compounds to be x = 0.15. The electrical conductivity of the compounds was enhanced depending on the level of the composition x. The highest value of 2.2 × 10−2 S cm−1 was obtained for x = 0.15, at 250 °C. It was suggested that the interstitial proton of the Sn1−x(Zn2/3Sb1/3)xP2O7 lattice was introduced by the formation of oxygen vacancies that originated from the cosubstitution of Zn and Sb for Sn. It was confirmed that the introduction of the interstitial proton significantly enhanced the electrical conductivity of the Sn1−x(Zn2/3Sb1/3)xP2O7 compounds. Thus, the Sn1−x(Zn2/3Sb1/3)xP2O7 compounds can be a promising candidate electrolyte with proton conducting at intermediate operating temperatures.