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The T–x phase diagram of (1 − x) [0.7PbZrO3–0.3(K0.5Bi0.5)TiO3]– xSrTiO3 (x = 0–0.7) solid solutions has been plotted using data from x-ray and dielectric studies. It was found that compounds with x < 0.25 and 0.25 < x ⩽ 0.70 undergo diffuse phase transitions to ferroelectric rhombohedral and tetragonal phases, respectively. The diffuseness of phase transitions is due to the ionic radii difference of cations which occupy positions in the A sublattice of perovskite ABO3 type structure. The study of the dielectric constant dispersion (10−1–3 × 106 Hz) revealed a wide spectrum of relaxation times. The average relaxation time follows the Arrhenius law with temperature dependent activation energy U*(T). It was found that the U*(T) dependence weakened as the diffuseness of the phase transition was increased.