The response characteristics of a proton pumping gate FET (PPG-FET) with a triple layer gate structure (Pd/proton conducting polymer/Pt) to various gases were investigated. Two kinds of hydrogen response signals (DC and AC) were obtained from the PPG-FET when the AC bias voltage was applied between the gate electrodes of the PPG-FET. The DC signals showed high hydrogen sensitivities and the sensitivities were about -30 mV/decade. Additionally, the AC signal decreased by 10 mV with increase in the hydrogen concentration from 100 ppm to 10%. Whereas the DC signals decreased slightly with increasing oxygen concentration, the oxygen response was much smaller than the DC hydrogen sensitivities. In contrast, the AC signal hardly responded to oxygen. This is because a proton conducting polymer layer prevents the movement of oxygen ions. Additionally, the proton pumping gate FET showed good selectivity against other gases (methane, ethane, ammonia, and methanol). These results clarify the high hydrogen sensitivity of the PPG-FET in an AC modulation mode. © 2009 The Institute of Electrical Engineers of Japan.