The fabrication of microsystems in which chemical measurements play a role make the on-chip combination of chemical sensors and signal processing circuitry highly desirable. Chemical sensors use sensitive layers which are not a part of MOS devices and in this sense, the determination of the best sensors and technological processes to achieve good integrated smart sensors is very important. In this paper a study of compatibility of ISFET chemical sensors and CMOS circuitry is presented for application to biomedical microsystems.