a chemical-sensing integrated-circuit suitable for the measurement of ionic species in aqueous solutions is described: it utilizes a chemical-sensitive field-effect transistor (chemfet) as an integral element in an analogue electronic circuit, so as to provide an active (i.e. operational) chemical microtransducer on a 2.03*2.43 mm silicon chip. the operational transducer possesses several important advantages over discrete chemfet devices, including a very small thermal sensitivity (0.024 mv degreesc/sup -1/), a wide range, linear functional response and an immunity to instabilities caused by reference electrode potential transients. it can be fabricated using exactly the same silicon process as a discrete chemfet device, without any significant increase in either the area of cost of the chip, and is well suited for adaptation to the measurement of a variety of chemical substances, including ions, enzymes, organic vapours and liquids, gases and immunological components, provided that the appropriate electroactive materials are available