Electroforming of an Al/Al2O3/polymer/Al resistive switching diode is reported. Electroforming is a dielectric soft-breakdown mechanism leading to hysteretic current–voltage characteristics and non–volatile memory behavior. Electron trapping occurs at early stages of electroforming. Trapping is physically located at the oxide/polymer interface. The detrapping kinetics is faster under reverse bias and for thicker oxides layers. Thermally detrapping experiments give a trap depth of 0.65 eV and a density of 5x1017 /cm2. It is proposed that the trapped electrons induce a dipole layer across the oxide. The associated electric field triggers breakdown and ultimately dictate the overall memory characteristics.