In this paper we present a method to obtain the electroaffinity and the ionization potential of conjugated polymers via Fowler-Nordheim tunneling current measurements. The method allows the measurement of the electroafinity and ionization potential for a sandwich type device! when the barriers at both sides of the polymer are different. The advantage of this method is that barrier height, the electroaffinity, and the ionization potential are obtained independent of the polymer film thickness and the electron (or hole) effective mass. We have used the proposed methodology to obtain the barrier heights and electroaffinity of poly (p-phenylene-vinylene) (PPV) in Metal/Polymer/Metal sandwich structures. For the present case the electrodes (Al/Au, Al/Ga, Al/In, Al/Sn) used lead only to electron and not hole tunneling current, thus only the electroaffinity has been obtained. The experimental value for the electroaffinity of PPV is of 3.87+/-0.04 eV.