The occurrence of phosphorus antisite defect centers on cation sites in GaP, CdSiP2, and ZnGeP2has been demonstrated by electron spin resonance. The paramagnetic defect is identified by its characteristic hyperfine and ligand hyperfine interaction, representative for a31P31P4cluster. The possible role of the antisite lattice defect as a center of nonradiative recombination in LEC‐grown GaP is emphasized.