In this paper we describe our investigations of the electrical conductivity of the silicon dioxide-air interface. It appears that this conductivity is caused by the adsorption of water vapour on the oxide surface and strongly depends on the relative humidity of the surrounding air. Considering this fact we have decided to investigate the possibility to reduce the surface conductivity by means of a chemical modification of the oxide surface, which reduces the adsorption of water vapour on it. To measure the conductivity we have used a so-called open-gate FET structure. The performance characteristics of this structure and the experimental results obtained with it are presented. We have found that by using silane agents like HMDS and DCDMS the surface conductivity of silicon dioxide can be reduced with at least a factor 1000 and 10000 respectively.