The effects of the substitution of Sn4+ at Ti4+ sites on the structural characteristics and microwave dielectric properties of (Mg0.95Ni0.05)(Ti1-xSnx)O-3 (MNTS, 0 <= x <= 0.25) ceramics were investigated. Single-phase ilmenite was confirmed up to x = 0.15, and then SnO2 was observed as secondary phase upon further substitution. For the single-phase specimens, the unit-cell volume increased with Sn4+ substitution due to the larger ionic radius of Sn4+ than TO4+. The quality factor of the MNTS specimens depended on their B-site bond valence. The dielectric constants of the specimens decreased with increasing Sn4+ substitution due to the lower polarizability of Sn4+ compared to Ti4+. The temperature coefficient of the resonant frequency of the specimens decreased with Sn4+ substitution, which could be attributed to the associated decrease in the tolerance factor of the specimens. The relationship between the microstructure and the microwave dielectric properties were also investigated. (C) 2014 Elsevier Ltd. All rights reserved.