Copper‐diffused InP and GaAs have been studied using junction space‐charge spectroscopy. Two dominant copper‐related deep level defects are observed in both materials. Photoionization cross sections of holes and electrons, thermal hole emission rates, and one electron capture cross section have been measured. In InP one of the ionization spectra exhibits a pronounced phonon‐related structure. The results, together with previous work on GaAs:Cu, GaP:Cu, GaAs:Cu, and AlGaAs:Cu, indicate a common geometrical structure for each of the two copper‐related defects in all the materials.