Experimental results are presented which shed light on the properties of thin discontinuous metal films as chemical sensing elements. They demonstrate further some interesting differences between hydrophilic and hydrophobic sensing structures. Two different methods are used to study the migration of charges out onto the oxide surface outside the metal gate of metal-oxide-semiconductor capacitors after treatment of the surface with hydrochloric acid. The charge migration is observed tither as a time dependent increase of the inversion capacitance or as a possibility to generate a photocapacitive current by a chopped light beam hitting the oxide surface at a distance from the contact. It is concluded that the charge migration occurs only if the surface was hydrophilic before the HCl-treatment. For a hydrophobic surface neither the inversion capacitance nor the photocapacitive current changed upon ion-treatment, An explanation for an observed increase of the capacitance level of hydrophilic structures immnersed in electrolytes is also given.