Modification of the microwave dielectric properties in Ba6-3xNd8+2xTi18O54 (x = 0.5) solid solutions by Bi/Sm co-substitution for Nd was investigated. A large increase in the dielectric constant and near-zero temperature coefficient combined with high ef values were obtained in modified Ba6-3xNd8+2xTi18O54 solid solutions where an enlarged solid solution limit of Bi in Ba6-3xNd8+2xTi18O54 was observed. Excellent microwave dielectric characteristics (epsilon = 105, Qf = 4110 GHz, and very low tau(f)) were achieved in the composition Ba6-3x(Nd0.7Bi0.18Sm0.12)(8+2x)Ti18O54.