Li salts (LiClO4 and Li2CO3) were used for diffusion under bias in polycrystalline diamond films. Hot-probe and conductance measurements were performed. A change in conductance of more than 10 orders of magnitude due to diffusion was observed. The p-type conductivity was obtained by conventional hot-probe measurements. The sign of the thermoelectric power was found to depend on the electric field normal to the surface. A hypothesis was advanced regarding the existence of a p-type inversion layer on the surface of the n-type hulk diffused layer.