Relaxation of photoconductivity is investigated in bulk GaP and GaN substrates and epitaxial layers as compared to porous GaP samples and GaN nanomembranes. Porous GaP samples with different characteristic sizes of pores and walls are produced by electrochemical treatment in substrates with different carrier concentrations, while GaN nanomembranes are produced by means of surface charge lithography. It was found that different mechanisms are responsible for persistent photoconductivity in nanostructured GaP and GaN. The photoelectrical properties of bulk and nanoporous GaP are explained on the basis of randomly distributed potential barriers due to the high Te doping level and partial compensation in bulk samples and by a porosity controlled potential barriers pattern in porous samples. Metastable defects were found to be responsible for the persistent photoconductivity in both bulk GaN layers and nanomembrane. Enhancement of the optical quenching of persistent photoconductivity was observed in GaN nanomembranes. The possible nature of point defects responsible for these effects is discussed.