A low-hysteresis voltage and high-sensing linearity chloride ion-sensitive sensor based on an extended gate field-effect transistor (EGFET) for real-time water quality monitoring microsystem applications is presented. All of the EGFET-manufacturing processes adopted in this work are compatible with standard integrated circuits planar technology, and therefore, they are very suitable for the mass production. Two EGFET-based chloride ion-sensitive microsensors having same channel width/length ratio (1000 μm/10 μm ) but with different channel geometries (rectangular and annular types) are presented. At pCl 3 (log[Cl − ]=−3 ) test point, a very small hysteresis voltage of the rectangular- and annular-channels EGFET-based Cl − microsensors (5 and 7 mV, respectively) can be achieved. As the concentrations tested ranging from pCl 1 (log[Cl − ]=−1 ) to pCl 5 (log[Cl − ]=−5 ), a very high-sensing linearity (99.23% and 99.08%) of the two types of EGFET-based Cl − microsensors is achieved. However, the sensitivity of the rectangular-channel EGFET-based Cl − microsensor (45 mV/pCl ) is much higher than that of the annular-channel EGFET-based Cl − microsensor (37 mV/pCl ). The selectivity coefficient of the investigated EGFET-chloride ion sensor under four different interfering ions (OH − , F − , SO 2− 4 , and Br − ) are also measured and analyzed.