The effect of Si4+ content on the microstructure and thermoelectric properties of Sr1 - xSixMnO3 - delta (0.05 <= x <= 0.3) samples is discussed. For x <= 0.15, the substitution of Si4+ for Se2+ leads to a significant increase in electrical conductivity and a decrease in the absolute value of the Seebeck coefficient due to an increase in electron concentration. For 0.2 <= x <= 0.3, the substituted Si4+ lowers the electrical conductivity. This is mainly attributed to decreases in the density and grain size as well as to the formation of secondary phases, such as Mn3O4, Mn2O3, and Sr2SiO4, resulting in a decrease in electron mobility. The substituted Si4+ reduces the thermal conductivity due to increased phonon scattering. The maximum dimensionless figure-of-merit (ZT) obtained in this study is 0.041 at 800 degrees C for x = 0.15. The partial substitution of Si4+ for Se2+ in SrMnO3 - delta yields a significant improvement in the thermoelectric properties. (C) 2015 Elsevier B.V. All rights reserved.