LaAlO3-stabilized La2/3TiO3 (LT) ceramics were prepared by the conventional mixed oxide route. Small amounts of manganese oxide were added to eliminate Ti4+ reduction. The powders were calcined at 1150 degrees C and sintered at 1400 degrees-1500 degrees C for 4 h and cooled at rates of 900 degrees-15 degrees C/h. The products were high density and single phase, with an average grain size of 6 mu m. The LaAlO3-stabilized LT ceramics exhibited a relative permittivity (epsilon(r)) of 64, a positive temperature coefficient of resonant frequency (tau(f)) of 84, and dielectric Q value x resonant frequency (Q x f) values of 16 400 GHz. The crystal structure and microstructures have been investigated using high-resolution transmission electron microscopy (HRTEM) in conjunction with X-ray diffraction (XRD). One candidate crystal structure, a approximate to 2a(p) (where a(p) is the lattice parameter of the high-temperature form of the cubic perovskite), b approximate to 2a(p), and c approximate to 2a(p) with a space group Cmmm (65), has been confirmed by XRD, electron diffraction, and lattice imaging techniques. Microtwins, with twin boundaries parallel to the {100} planes, were observed in the microstructures.