A comprehensive analysis of experimental data relating to so-called "soft sparking" mode of plasma electrolytic oxidation (PEO) has been undertaken. The transition to the soft sparking mode is accompanied by a number of characteristic effects, such as a decrease in anodic voltage, acoustic and light emission, increase in hysteresis in transient current-voltage curves, improved uniformity of the discharge distribution on the surface, disappearance of atomic lines, and development of continuous radiation in the optical emission spectra. An explanation of the main features of PEO process operated under soft sparking conditions is proposed assuming the existence of a specific narrow region in the coating thickness, where the main anodic voltage drops. Because of high electric field in this "active zone", both anodic oxidation of the metal substrate and high-energy processes may take place. According to this assertion, the soft sparking mode of PEO is caused by cathodic polarization (a) eliminating the potential barrier at the oxide-electrolyte interface due to local acidification and (b) increasing electric field at the metal-oxide interface during subsequent anodic half-cycle due to narrowing of low-conductive part within the active zone. Based on this consideration, it is possible to account for the main characteristic phenomena accompanying the PEO process on aluminum under alternating polarization.