Electrochemical Impedance Spectroscopy (EIS) was used to study the sensing mechanism of sulfide ion sensor assembled from CdS-bentonite. The sensor exhibited good response to sulfide ion with Nernstain slope closed to theoretical value, 29.5. The Nyquist plot obtained from EIS’s experiment indicated the capacitive behavior of the electrode. This could be due to the double layer on the surface. The positive charged which adhered on the negative charge layer of bentonite could induce the sulfide negative ion in the solution onto the electrode surface. Selectivity coefficient as well as Nernstain slope indicated that only sulfate ion could be an interfering ion while measure sulfide ion in solution, and easily prevented by conventional precipitation method. The sensor could be used to measure sulfide ion in the concentration range 1x10^-4 to 0.1 M, the appropriate pH range was 4 to 10. It was used to measure sulfide ion in waste water samples with good percent recovery.