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All chemical sensors based upon the field-effect principle share a common quality. Their measurable properties can be described in terms of a flat-band voltage. The various terms in the expression of the flat-band voltage are described and discussed, and in particular the voltage drop at the insulator-electrolyte interface. It is shown that this voltage drop depends on the of the electrolyte and is determined by two parameters, the at the point of zero charge, and a sensitivity parameter which is introduced in this paper. These parameters are obtained from the site-dissociation model of the insulator-electrolyte interface, combined with the Gouy-Chapmann-Stern theory of the electrical double layer at this interface. The theoretical description is used to interpret experimental results obtained from insulators with widely different properties, namely SiO ₂ and Al ₂ O ₃ .