High‐resolution electron energy loss spectroscopy, low‐energy electron diffraction, and quadrupole mass spectrometer have been employed to study the effect of atomic hydrogen on the acetylene saturated preadsorbed Si(100)(2×1) surface at room temperature. It is evident that the atomic hydrogen has a strong effect on the adsorbed C2H2and the change of the underlying surface structure of Si. The experimental results show that CH and CH2radicals coexist on the Si surface after the dosing of atomic hydrogen; meanwhile, the surface structure changes from Si(2×1) to a dominant of (1×1). These results indicate that the atomic hydrogen can open C=C double bonds and change them into C–C single bonds, transfer the adsorbed C2H2to C2Hx(x=3,4) and break the underlying Si–Si dimer, but it cannot break the C–C bond intensively. Some C4species have been formed during the dosing with atomic hydrogen. It may be the result of atomic hydrogen abstraction from C2Hxwhich leads to carbon catenation between two adjacent CC dimers. The formed C4is stable on Si(100) surfaces up to 1100 K and can be expected to host diamond nucleation. ©1995 American Institute of Physics.[S0003‐6951(95)04246‐X]