Surface phonons, electronic structure and chemical reactivity of the diamond (100)(2 x 1) surface have been studied using electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). Vibrational losses are observed at similar to 80, 92, 123, 135, 147 and 165 meV for a clean C(100)(2 x 1) surface. The 92 meV loss is assigned to the in-phase bouncing mode of the surface dimers. The origins of the other losses are discussed. Electronic transition is observed at 3.5 eV which is associated with the interband transition between the pi and pi* surface states. The chemical reactivity of the C(100)(2 x 1) surface towards several gases, H, H-2, O, O-2, CO, N2O and C2H2, has been investigated at 90 and 300 K. The chemical reactivity of the C(100)(2 x 1) surface towards these gases is compared with that of the Si(100)(2 x 1) surface, and the origin of the difference in the reactivity is discussed.