We measured the dielectric susceptibility and the polarization of three crystals in the system BaxSr1−xTiO3(x=0.39, 0.26, andx=0.24) which exhibit a diffused ferroelectric phase transition. The results are interpreted using an extension of the Devonshire theory to diffused phase transitions. Diffuseness of the phase transition in BST is not caused by crystalline imperfections but is an inherent feature of this transition.