Disclosed is an ISFET comprising a H.sup.+ -sensing membrane consisting of RF-sputtering a-WO.sub.3. The a-WO.sub.3 /SiO.sub.2 -gate ISFET of the present invention is very sensitive in aqueous solution, and particularly in acidic aqueous solution. The sensitivity of the present ISFET ranges from 50 to 58 mV/pH. In addition, the disclosed ISFET has high linearity. Accordingly, the disclosed ISFET can be used to detect effluent.