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This study aims to investigate the microwave properties of coplanar waveguide (CPW)-based poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) conducting polymer line in an ethanol gas atmosphere, with the frequency range of 0.5–2 GHz. For an ethanol-exposed PEDOT:PSS line (test sample), the transmission coefficient (<i>S</i>₂₁) decreased immediately; moreover, the microwave effective conductivity (σ<i>_m/w</i>) decreased simultaneously, compared with the ethanol-free PEDOT:PSS line (reference sample). The immediate variations in Δ<i>S</i>₂₁ ( = <i>S</i>_21,ethanol − <i>S</i>_21,free) and Δσ<i>_m/w</i> ( = σ<i>_m/w</i>_,ethanol − σ<i>_m/w</i>_,free) were approximately 10.2 dB and 2.7 × 10⁴ S/m, respectively. Furthermore, in the analysis of the circuit model of the PEDOT:PSS line, the characteristic impedance and distributed elements, i.e., resistance (<i>R</i>) and inductance (<i>L</i>) per length, of the test sample increased, compared with the reference sample. However, upon stopping the exposure to ethanol gas, the microwave properties of the test sample instantaneously recovered to those of the reference sample. According to these critical observations, we could confirm that the coplanar waveguide with a PEDOT:PSS line shows a significant difference in the diverse microwave properties, through rapid response to the ethanol gas at room temperature.