The effect of TiCl3 and 0.5, 1.0, 2.0 mol% B2O3 addition on the microstructure and on the densification of Ti-excess BaTiO3 were investigated. Sb2O3 was used as a donor dopant in boron containing compositions. The Ti-excess, platelet-type Ba2Ti5O12 or Ba4Ti12O27 phases were detected in undoped and Ti-excess formulations and compositions doped with 0.5 mol% B2O3, which were sintered below the eutectic temperature. Suppression of these platelet-type grains occurred in the formulations containing more than 0.5 mol% B2O3. Donor doping by Sb2O3 was very effective for promoting twin formation due to the increased concentration of the Ti3+ ions. Sudden grain growth occurred at the 2 mol% B2O3 addition level, with the loss of twin lamellae within the structure. The addition of 0.2 mol% excess TiO2 in liquid form (TiCl3) to the 0.5 mol% B2O3 containing sample, reduced the size and the number of polygonal grains and had a marked effect on the twin formation due to the homogeneous distribution of the excess Ti ions in the composition. The effects of adding the grain boundary modifier B2O3 (before and after the calcination step) on the microstructure and on the positive temperature coefficient of resistivity (PTCR) of Sb2O3 donor-doped Ti-excess barium titanate (BT) were also investigated; PTCR effects up to 5.0 orders of magnitude were obtained. (c) 2006 Elsevier Ltd. All rights reserved.