In this paper is described how hydrogel is exploited as sensor material for the detection of carbon dioxide (CO2). A pH-sensitive hydrogel disk, which swells and deswells in response to pH changes, was clamped between a pressure sensor membrane and a porous metal screen together with a bicarbonate solution. CO2 reacts with the bicarbonate solution resulting in a pH change, which is converted into a pressure by the enclosed hydrogel. This pressure is a measure for the partial pressure of CO2. The main advantage of this sensor principle is the fact that a reference electrode as required for potentiometric sensors is no longer needed.