Tin (Sn) substitution into the B-site and Nd/Sn cosubstitution into the A- and B-sites were investigated in a Ba6-3Sm8+2xTi18O54 solid solution (x = 2/3). A small amount of tin substitution for titanium improved the temperature coefficient of resonant frequency (tau(f)) but led to a decrease of the relative dielectric constant (epsilon) and the quality factor (Qf). The Ba6-3xSm8+2(Ti1-zSn.)(18)O-54- based tungsten-bronze phase became unstable for compositions with a tin content of greater than or equal to10 mol%, where BaSm2O4 and Sm-2(Sn,Ti)(2)O-7 appeared, and finally, these phases became the major phases. On the other hand, Nd/Sn cosubstitution led to a good combination of high epsilon, high Qf, and near-zero tau(f) Excellent microwave dielectric properties were achieved in Ba6-3X(Sm1-yNdy)(8+2x)(Ti1-zSnz)(18)O-54 ceramics with y = 0.8 and z = 0.05 sintered at 1360degreesC for 3 h: E = 82, Qf = 10 000 GHz, and calculated tau(f) = + 17 ppm/degreesC. The tolerance factor and electronegativity difference exhibited important effects on the microwave dielectric properties, especially the Qf value. A large tolerance factor and high electronegativity difference generally led to a higher Qf value.