The solid solubility of R ions (R = Ho3+, Dy3+, and Y3+) in the BaTiO3 perovskite structure was studied by quantitative electron-probe microanalysis (EPMA) using wavelength-dispersive spectroscopy (WDS), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). Highly doped BaTiO3, samples were prepared using mixed-oxide technology including equilibration at 1400degrees and 1500degreesC in ambient air. The solubility was found to depend mainly on the starting composition. In the TiO2-rich samples a relatively low concentration of R incorporated preferentially at the Ba2+ lattice sites (solubility limit similar toBa(0.986)R(0.014)Ti(0.9965)(V-Ti'''')(0.0035)O-3 at 1400degreesC). In BaO-rich samples a high concentration of R entered the BaTiO3 structure at the Ti4+ lattice sites (solubility limit -BaTi0.85R0.15O2.925(V-O(..))(0.075) at 1500degreesC). Ho3+, Dy3+, and Y3+ incorporated preferentially at the Ti4+ lattice sites stabilize the hexagonal polymorph of BaTiO3 The phase equilibria of the Ho3+-BaTiO3 solid solutions were presented in a BaO-Ho2O3-TiO2 phase diagram.