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Angle-resolved ultraviolet photoelectron spectroscopy has been used to determine the initial-state energy versus k~~dispersion of surface states on the Si(100) (2X 1), twodomain, reconstructed surface. One surface state was found at 0.70 eV below EF at the I symmetry point. The energy dispersion relation for this surface state was measured along the I -J, I -J', and J -K symmetry lines of the (2& 1) surface Brillouin zone (SBZ). A second surface state was observed in the energy range between 2 and 3 eV below EF. The energy dispersion relation for this surface state was obtained along the J -E symmetry line of the SBZ and in the [010] bulk azimuthal direction. The experimentally obtained dispersion relations are compared with the surface electron bands as given by a tightbinding calculation of the asymmetric dimer model. The experiment gives a surface-state band whose center of gravity is in good agreement with the calculation, while the bandwidth is approximately half as large as the calculated. For the lower surface state both initial energy position and dispersion are in good agreement with the calculation. The upper surface state is assigned to a dangling-bond band and the lower is tentatively assigned to the dimer bond.