A hybrid furnace, which allows the simultaneous application of microwave and radiant energy, has been used to investigate the sintering of partially stabilized zirconia (doped with 3 mol % yttria). Microwave-enhanced sintering is clearly demonstrated with densification occurring at a lower temperature when a high-frequency electric field is applied. By considering the variation of electric field strength with furnace temperature, this enhancement is shown to be non-thermal in nature, being dependent on the electric field strength and not the power density (heating) of the microwaves. This dependence on electric field is consistent with an additional driving force term in the equation which describes the diffusion of vacancies through the material during sintering.