A new capacitive field-effect microsensor based on a porous EIS (electrolyte-insulator-semiconductor) structure is presented. The porous silicon sensor was prepared using standard techniques of semiconductor processing. A well-defined macroporous layer was formed on silicon by electrochemical etching and a SiO2/Si3N4 sandwich was deposited as insulating and pH-sensitive layer. The porous sensor exhibits a high, near-Nernstian pH sensitivity of about 54 mV per decade in the concentration range from pH 4 to pH 8, similar to a planar non-porous EIS structure with the same layer sequence. The enlargement of the active sensor area (surface) due to the porous structure increases the measured capacitance and thus allows a scaling down of the sensor. The preparation of biosensors based on the same structure is demonstrated by immobilization of the enzyme penicillinase as biosensitive component.