It is well known that evaporated metal-silicon oxide-metal sandwich structures exhibit a field-assisted lowering of the potential barrier between donor-like centres and the conduction band edge, known as the Poole-Frenkel effect. This behaviour is indicated by a Linear dependence of logJ on V-1/2, where J is the current density and V is the applied voltage. Less well known is that when a suitable electrode metal is used, and after the application of a few volts under moderate vacuum conditions, the structure undergoes a process known as 'electroforming', after which characteristic features such as voltage-controlled differential negative resistance (VCNR) and electron emission are exhibited. The work described in the present investigation is concerned with the d.c. properties during voltage cycling prior to electroforming. In agreement with previous work Poole-Frenkel conductivity was observed during the first voltage cycle, but the value of the Poole-Frenkel field-lowering coefficient beta generally increased with the number of voltage cycles before the onset of electroforming, frequently attaining a value of up to twice the theoretical value. No significant variation in the value of beta was observed in samples which did not subsequently electroform. Enhanced beta values have previously been identified with the existence of a non-uniform electric field region within the insulator, and it was therefore concluded that a high-held region is established during the voltage cycling, which is necessary before electroforming can take place.