Atomic-layer adsorption of P on Si(100) and Ge(100) at 200-750 degrees C by PH3 was investigated using an ultraclean low-pressure chemical Vapor deposition (CVD) system. At 300 degrees C, the PH3 adsorption was suppressed on the H-terminated Si surface, but PH3 was adsorbed dissociatively on the H-free Si surface with saturation tendency to subatomic layer. At 450-750 degrees C, the P atom concentration on the Si surface tended to saturate to about two or three atomic layers by exposing PH3 with little influence of the carrier gas (H-2 or He). When the P-adsorbed Si was kept in Ar and in H-2 at 650 degrees C after PH3 exposure, the P atom concentration decreased to about one atomic layer by thermal desorption and also by reduction due to hydrogen. On the Ge surface, PH3 adsorption was suppressed by N-termination at 200 degrees C, P atom concentration saturated to the single atomic layer at 300-450 degrees C. Furthermore, P desorption from the Ge surface at 450 degrees C occurred much faster than that from the Si surface at 650 degrees C, while P bonded to Ge was stable at 300 degrees C.