The frequency and temperature dependences of components of the impedance of Pb_0.75Sn_0.25Te:In single crystals are studied within the frequency range from 20 to 10⁶ Hz and at temperatures from 4.2 to 300 K. Impedance spectra are analyzed using the equivalent-circuit model. It is shown that at T = 4.2 K the impedance spectrum of a sample is consistent with that of the equivalent circuit of a parallel RC network. The calculated circuit parameters yield E ≈ 1000 for the permittivity of the material at low temperatures. An increase in free-carrier concentration induced by either heating of the sample or by its illumination (at temperatures below T _c ≈ 25 K) gives rise to a substantial increase in the capacitive contribution to conductivity. It is shown that the increase in the capacitive contribution to conductivity cannot be associated with a phase transition. The possibility is considered that the increase in the capacitive contribution is related to the charge exchange processes that occur in the impurity subsystem and are induced by the ac electric field.