The surface chemistry of Si during gas-source molecular beam epitaxy (GSMBE) doped with phosphorus (P) has been investigated in detail, by obtaining the hydrogen (H) and P coverages on the growing surface by silane (SiH4) and phosphine (PH3). The low temperature region of doped epitaxy was found to consist of two domains: domain I (550 < T < 650 degreesC) rate limited by P desorption and domain II rate limited by both H and P desorptions. Growth rates in the high temperature region (T > 650 degreesC) increased with mild P dopings. which is related to enhanced sticking probability at Si sites in the presence of surface P. The P coverage on the growing surface was some three orders of magnitude greater than that in the bulk, suggesting rapid surface segregation as a major cause for doping limitation.