The influence of metal/diamond interfacial nanostructure and diamond surface treatment on electron emission from diamond-coated Mo emitters is presented. Diamond coatings are known to enhance electron emissivity but may do so at the interfacial barrier and/or the surface barrier, since both influence the magnitude of the current. Prior to annealing the energy distribution of the emitted electrons exhibits a linear displacement in peak position with applied voltage. After annealing in a hydrogen plasma at 600 degrees C, emission is further enhanced and the magnitude of the peak shift with applied voltage is reduced. A further oxygen plasma treatment yields an intermediate dependence of peak shift with voltage. The effects of hydrogen plasma annealing appear closely related to changes in the nanostructure at the Mo/diamond interface. During annealing Mo2C particles are formed while both molybdenum oxides and an amorphous layer present on the original diamond particles are removed. The smaller voltage drop after the oxygen plasma treatment is thought to be due to the removal of passivating hydrogen as well as to a change in the surface barrier of the diamond. The energy barrier of the diamond/vacuum interface plays only a minor role in the voltage drop.