Random phthalide-containing poly(arylene ether ketones) have been synthesized. The properties of the polymers have been studied by the thermostimulated-current and thermostimulated-depolarization methods. An extremal dependence of the electrophysical properties on the content of phthalide groups in macromolecules is observed with a maximum at their concentration of 10-15%. These dependences are explained in terms of the Sworakowski model in which the role of polarization energy of phthalide-containing molecular fragments is distinguished. The feasibility of targeted variations in the electrophysical properties of nonconjugated polymers through incorporation of functional blocks into them is discussed.