We applied slow positrons to both as‐etched GaP and (NH4)2Sx‐treated GaP. The results show that the surface of as‐etched GaP is active for the adsorption of oxygen atoms when the etched surface was exposed to air for several minutes before the measurement. On the other hand, the monolayer of chemisorbed sulfur in (NH4)2Sx‐treated GaP is effective to protect the clean surface from the adsorption of the oxygen atoms. The mean diffusion length of positrons in the etched GaP is shorter than that in (NH4)2Sx‐treated GaP. This suggests that the centers for the positron trapping, such as Ga vacanciesVGaand/orVGa‐related complexes, are created by the adsorption of oxygen atoms.