Recent work has established that in evaporated SiOx thin film sandwich structures with gold electrodes, which show the Poole-Frenkel effect (enhanced conductivity at high electric fields due to the lowering of the potential barrier at donor-like centres), the value of the Poole-Frenkel field-lowering coefficient beta increases with voltage cycling prior to the onset of electroforming. These enhanced beta values were associated with the establishment of a high-field region during the electroforming process. In the present work, aluminium electrodes were used in order to explore the characteristics of contrasting system. Electroforming occurred in some samples, but with a maximum current value of less than 1 mA, considerably less than with the gold electrodes. Poole-Frenkel conductivity was observed in the initial voltage cycles, with a beta value of typically 4.5 X 10(-5) eV m(1/2) V-1/2, moderately exceeding the theoretical value. Electroforming normally took place after several voltage cycles, but was not permanent, with a reversion to Poole-Frenkel conduction during some cycles. As for the case with gold electrodes, the value of beta increased in the first few cycles, but after further voltage cycling, the behaviour became less stable with varying values of beta. After further voltage cycling, electroforming disappeared and beta reduced to the order of 2 X 10(-5) eV m(1/2) V-1/2. This behaviour can be understood in terms of a filamentary conduction model, in which the stability of samples with aluminium electrodes is inferior to those with gold electrodes, as a consequence of the lower melting point. (C) 2003 Elsevier Science B.V. All rights reserved.