We have produced epitaxial Fe overlayers on S-passivated GaAs(100) surfaces by CH3CSNH2 treatment. The correlation between magnetic properties of the overlayers and surface chemical structure of GaAs(100) surfaces was investigated by changing the annealing temperature of the surface prior to growth. The results show that the magnetization of Fe overlayers is crucially determined by the GaxS chemical bonds and by the presence of excess As after the anneals. A comparative investigation of the magnetization has been performed on both S-passivated and clean GaAs(100). It is confirmed that S-passivation on GaAs surfaces can effectively eliminate the magnetization deficiency previously attributed to interdiffusion of As into the Fe overlayer.