A method of selectively doping the polyaniline gates of chemically sensitive field-effect transistors (CHEMFETs) has been demonstrated. Activated and inactivated gates located on the same chip were exposed to a solution containing palladium. The activated gate was doped by palladium by the electroless relaxation process I,ut the inactivated gate was not modified. High selectivity of the doping process was proved by exposing the devices to hydrogen. The method has potential applications in manufacturing single-chip heterogeneous microsensor arrays.