A quick response NO2 sensing device has been developed based on an improved gas-permeable Au electrode and a polystyrene film formation in styrene gas plasma. This device has a sandwich structure; that is, the polystyrene film is between the Au substrate and the gas-permeable Au electrode. An ionic current passing through the film changes rapidly (within a few seconds) and is reversible when this film is exposed to a 2% (v/v) NO2 gas mixture. This rapid change of current results from using an oblique deposition technique on the gas-permeable electrode. To prevent delamination of the film from the substrate, d.c. voltage (15 V) was applied between parallel Cu back plates attached to the Au substrate in the plasma and exited by r.f. 1 MHz.