The electronic structure of the diamond C(100)-(2×1)/(2×2) has been investigated by means of angle-resolved photoelectron spectroscopy for the first time. A surface-related shift has been observed in the C 1s core-level spectrum. The surface-state band dispersion was measured along the symmetry axis Γ–J′ in the surface Brillouin zone. For k∥=0, there is a very pronounced surface state 1.5 eV below Fermi level EF , and it disperses downwards with increasing k∥. Near the boundary of the surface Brillouin zone J′, we find two states with binding energies of 1.9 and 2.4 eV with respect to EF .