In this paper the electrochemical impedance response of phase transition materials is investigated within the Cahn–Hilliard framework. The application of regular perturbation techniques allows the determination of analytical expressions for stable perturbations around constant initial condition. If the initial condition corresponds to a steady state configuration with coexistence of two phases on the sample, then approximate analytical expressions can be found by suitable modification of the boundary conditions. In particular, the impedance response of a particle being synchronously charged/discharged on both sides (reflective conditions), featuring two phases and a phase transition region, is equivalent to the impedance response of a particle of smaller size allowing current go through (absorbing conditions).