Many sensors proposed to date require high operation temperature (e.g., 400°C for solid electrolytes) to detect with a high sensitivity, which restricts the wide application of sensors. We have found that composites consisting of the emeraldine base‐polyaniline (EB‐PAn) and poly(vinyl alcohol) (PVA) serve as a promising sensor operating at room temperature with a high sensitivity. The logarithm of electrical conductivity of the EB‐PAn/PVA composite is proportional to the log of concentration. For the composite with 13 wt % EB‐PAn and 87 wt % PVA, the linear relationship holds in the concentration range from 50 ppm to 5% at 30% relative humidity. This composite is insulating in a moist atmosphere without . With the addition of , however, carbonate ions are formed by the hydrolysis of , and these ions equilibrate with the atmospheric . The carbonic acid is reversibly incorporated to and ejected from the EB‐PAn depending on the concentration of atmospheric , and the conductivity varies between conducting and insulating levels. © 2000 The Electrochemical Society. All rights reserved.