Electrical forming and voltagecurrent characteristics of AupolystyreneAu (MIM) structures created on aglassy substrate were studied. Polystyrene (PS) isolating films, 180800nm thick, were prepared by spinning from asolution. It was shown that the electroforming of MIM structures occurs at the voltage of 66.5V and that the shaping rate depends on the insulator thickness, temperature, and the shaping time. The voltagecurrent characteristics exhibit aregion with ohmic behaviour and, above acertain voltage, aregion with effective negative resistance. The negative resistance is explained by deterioration of conductive Au filaments at higher voltages. The voltage decrease does not result in acomplete recovery of all filaments.