Positive temperature coefficient of resistivity (PTCR) ceramics of 0.912(Ba1+xTiO3)–0.088(Bi0.5Na0.5TiO3) (BT–BNT) (x = −0.03 to 0.03) were prepared using the mixed oxide route and sintered at 1340°C for 4 h. Products were predominantly single phase with a tetragonal structure and grains in 2–6 μm size containing 90° ferroelectric domains. Samples with Ti/Ba > 1 contained second-phase Ba6Ti17O40. HRTEM and aberration-corrected Z-contrast high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) suggested that the dopants, Bi and Na, occupy the Ba site of the perovskite lattice, and revealed the presence of dissociated dislocations in x = −0.03 and x = 0.00 materials. The interval between two partial dislocations was 1.9–3.4 nm, yielding stacking fault energies of 363–649 mJ/m2. The PTCR behavior of the ceramics increased with Ti/Ba content, reaching a maximum of six decades change in resistivity for x = −0.03. The anomalous increase in resistivity depends critically on stoichiometry, increasing with the Ti/Ba ratio; this in turn is directly correlated with an increase in the amount of second-phase Ba6Ti17O40, an increase in the stacking fault energy, and an increase in the tilt angle of the grain boundaries.