A theory of the ac surface photovoltage is presented for the case of surface recombination of carriers via mobile centers located in the insulator in the region adjacent to the interface. The surface photovoltage relaxation time for this recombination mechanism is shown to be strongly dependent on the modulation frequency of the incident light, while the relaxation time is frequency independent if the recombination of carriers occurs via conventional, spatially fixed, surface states. The dependence of the ac surface photovoltage on the modulation frequency of the incident light was measured forn‐Si/native oxide andn‐GaAs/native oxide systems. The strong frequency dependence of the surface photovoltage relaxation time observed for ann‐Si/native oxide system under depletion conditions indicated that the carrier recombination at this interface occurs mainly via mobile oxide centers. The relaxation time inn‐GaAs/ native oxide system was found to be frequency independent, confirming previous conclusions that surface states in GaAs are associated with defects located on the semiconductor side of the interface.