A plant for drying and solid state polycondensing a moisture-containing polymeric material, which plant comprises: - a predetermined amount of moisture-containing polymeric material in granular form; - an inlet station for loading said moisture-containing polymeric material in granular form; - an outlet station for discharging the dried and solid state condensed polymeric material; - a conduit for feeding the material to be treated in a predetermined longitudinal direction and located between said inlet station and said outlet station; - at least one treatment zone for treating the polymeric material, which is located along said conduit; - radiating means for emitting an alternating electromagnetic field in the radio-frequency band, for thermal treatment of the polymeric material, such that it can be dried and upgraded; wherein said radiating means comprise a plurality of applicators located in correspondence of said treatment zone and external to said conduit, in longitudinally offset positions, said applicators being connected to the terminals of an electromagnetic wave generator; and wherein said applicators comprise pairs of opposed radiating elements (11, 11') each connected to a respective terminal (14, 16) of said wave generator to provide said alternating electromagnetic field having field lines at least partially parallel to the direction of feed of the material and define magnetic dipoles with opposite polarities along said conduit; wherein said radiating elements (11, 11') consist of rings made of a conductive material and substantially perpendicular to said longitudinal direction of said conduit; wherein said conduit comprises a side wall made of a dielectric material which extends in a substantially vertical direction to facilitate natural gravity feed of the material, characterized in that said conduit has an inside diameter ranging from 20 mm to 60 mm, said rings comprising a first series of rings connected in parallel to one of the terminals of said generator and a second series of rings connected in parallel to the other terminal of said generator, said series (13, 15) being in alternate arrangement, an inert gas selected from the group consisting of argon and nitrogen, blower means being provided to blow said inert gas into said conduit to facilitate removal of the moisture that has come out of the polymeric material upon drying, said conduit, said inlet station and said outlet station being isolated from the external environment to maintain a controlled atmosphere therein. - a predetermined amount of moisture-containing polymeric material in granular form; - an inlet station for loading said moisture-containing polymeric material in granular form; - an outlet station for discharging the dried and solid state condensed polymeric material; - a conduit for feeding the material to be treated in a predetermined longitudinal direction and located between said inlet station and said outlet station; - at least one treatment zone for treating the polymeric material, which is located along said conduit; - radiating means for emitting an alternating electromagnetic field in the radio-frequency band, for thermal treatment of the polymeric material, such that it can be dried and upgraded;