Many CO2 sensors proposed to date require high operation temperature (e.g., 400 degreesC for solid electrolytes) to detect CO2 with a high sensitivity, which restricts the wide application of CO2 sensors. We have found that composites consisting of the emeraldine base-polyaniline (EB-PAn) and poly(vinyl alcohol) (PVA) serve as a promising CO2 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 CO2 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 CO2. With the addition of CO2, however, carbonate ions are formed by the hydrolysis of CO2, and these ions equilibrate with the atmospheric CO2. The carbonic acid is reversibly incorporated to and ejected from the EB-PAn depending on the concentration of atmospheric CO2, and the conductivity varies between conducting and insulating levels.