The electronic structure of the diamond C (100)-(2 X 1)/(2 X 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 Is core-level spectrum. The surface-state band dispersion was measured along the symmetry axis GAMMA-J' in the surface Brillouin zone. For k(parallel-to) = 0, there is a very pronounced surface state 1.5 eV below Fermi level E(F), and it disperses downwards with increasing k(parallel-to). 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 E(F).