A chemical selective sensor system utilizes admittance modulated to detect the presence of chemicals or chemical species in a fluid. The sensor system includes a film or membrane adapted to pass ions when selected chemicals which are to be detected are present at the membrane surface. The membrane is attached to a hydrophilic layer of material which, in turn, is attached to a transformed layer which is deposited on a base substrate. When the selected chamicals are present in the fluid, the membrane interacts with the chemicals to allow ions, also in the fluid, to permeate the membrane. This ion current in the membrane is transformed or converted by the transformer layer to an electronic current which is measured by an electrical circuit coupled to the transformer layer of material. An alternating current source is coupled at one terminal to the transformer layer and to the measuring circuit, and at another terminal to a circuit return electrode.