- The investigation of the behavior of the ferroelectric phase transition with magnetic field tuning and concentration change is highly attractive owing to the possibility to prepare alloying samples and to predict theoretically the parameters of the magnetic field and concentration response at relatively small concentrations. These parameters may be extracted from the equation of states of the perovskite under investigation in the assumption of the linear response. The study of the movement of the paraelectric-ferroelectric interphase boundary in (Ba,Sr)TiO3 with concentration change and in constant magnetic fields is provided in the framework of the mean-field theory. The analytical solution for the parameters of motion of the interphase boundary is applied to the calculations of the splitting of domain walls in (Ba,Sr)TiO3 with the magnetic field tuning for different concentrations of Sr. The calculations are based on the experimental data for the Curie-Weiss constant and for the parameters of the Landau-Ginzburg expression for the free energy.