The tungstenbronze-type like Ba6-3xR8+2xTi18O54 (R = rare earth) solid solutions, which have excellent microwave dielectric properties, have been investigated about the effect of substituting Bi ions on the lattice parameters and the phase structure. The x-values for the Ba6-3x(R(8+2x)-yBiy)Ti18O54 formula were fixed at x = 2/3, and the y-values were varied from 0.0 to 0.3. In the case of La and Nd series which are widely used in the dielectric resonator applications, it seems that until y = 0.05 Bi ions substituted for Ba ions, and thereafter for La or Nd ions from y = 0.05 to 0.3. In the case of La and Nd series, the dielectric constant (epsilon(r)) decreased in the early stage of 0 < y < 0.1 and 0 < y < 0.05. (C) 2002 Elsevier Science B.V. All rights reserved.