High-temperature phase relationships in different ternary oxide systems were investigated using X-ray diffraction and electron microscopy (SEM) with energy- and wavelength-dispersive spectroscopy (EDS and WDS). An unstable perovskite La2/3TiO3 compound was identified in the systems La2O3-TiO2-Al2O3, La2O3-TiO2-Ga2O3, La2O3-TiO2-Nb2O5 and Nd2/3TiO3 in the system Nd2O3-TiO2-Al2O3. In all these systems the third oxide added actually stabilizes the R2/3TiO3 (R=La,Nd) compound. In contrast, the additon of BaO to La2O3 -TiO2 or Gd2O3-TiO2 does not stabilize this compound but leads to the formation of a perovskite-like solid solution with the formula Ba6-xR8+2x/3Ti18O54. The extent of the stability range of these solid solutions was determined. Selected ceramics based on the perovskite compound were characterized in terms of their dielectric properties.