Two series of experiments were performed to study the experimental conditions for the formation of {111} twins and related microstructures in barium strontium titanate ((Ba,Sr)TiO3). In the first series, the phase equilibria in the BaTiO3-SrTiO3-TiO2 system were determined. XRD and WDS analysis, done in the BaTiO3-rich region, of 45(Ba,Sr)TiO3-10TiO(2) samples annealed at 1250degreesC for 200 h in air showed that (Ba,Sr)TiO3 was in equilibrium with Ba6Ti17O40 (B6T17) and Ba4Ti13O30 phases with strontium solubility (Sr/(Ba + Sr)) of similar to0.02 and 0.20, respectively. In the second series the micro-structures of samples consisting of a mixture of (Ba,Sr)TiO3 and 2.0 mol% TiO2, were observed after sintering at 1250degreesC for 100 h in air. {111} twins formed only in the samples with faceted B,T,, second phase particles, similar to the case of BaTiO3. In these samples, abnormal grain growth occurred in the presence of the {111} twins. In contrast, no {111} twins formed and no abnormal grain growth occurred in the samples containing second phase particles other than B6T17. With an increased substitution of strontium for barium, the aspect ratio of abnormal grains containing {111} twin lamellae was reduced. This result was attributed to a reduction in the relative stability of the {111} planes with the strontium substitution.