- The investigation of the behaviour of the ferroelectric phase transition with concentration change is highly attractive owing to the possibility of preparing alloying samples and to predict theoretically the parameters of the concentration response at relatively small concentrations. These parameters may be extracted from the equation of state of the perovskite under investigation in the assumption of a linear response. The utmost sensitivity of ferroelectric properties to concentration change is the well known from experiments as well as first-principles calculations. One of the most pronounced concentration effects on the ferroelectric properties is the large shift of phase transition temperatures with doping. The study of the movement of the paraelectric-ferroelectric interphase boundary in PB(Zr, Ti)O3 with concentration change is provided in the framework of the mean-field theory. The analytical solution for the parameters of motion of the interphase boundary is applied for the calculations of the width and velocity of the latter at different concentrations of Zr. 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.