Microstructural investigations and microanalyses of a series of Ba6-xR8+2/3xTi18O54 ceramics (R = Nd, Gd) revealed that the solid-solubility limit for the isovalent substitution of R3+ by Bi3+ depends on the composition of the Ba6-xR8+2/3xTi18O54 phase. For the Nd analogue the solid-solubility limit (y in Ba6-x(Nd1-yBiy)(8+2/3x)Ti18O54) decreases with a decrease in x from y = 0.16 for x = 2.0 to y = 0.10 for x = 0.8. An even lower solid-solubility limit (y = 0.06) was found for the Ba-4.5(Gd1-yBiy)(9)Ti18O54 compound (x = 1.5). All substituted Nd compositions exhibit higher permittivities (93-99), lower temperature coefficients of permittivity (11-15 ppm/K) and higher dielectric losses (Q . f = 1300-5500 GHz) than the parent compositions. By exceeding the solid-solubility limit, abrupt changes in the microstructural and dielectric characteristics are induced.