High sensitivity chlorine (Cl-2) gas detection has been realized by newly developed gas sensors using Mg-phthalocyanine (MgPc) thin films. The MgPc thin film gas sensors exhibited an increase of conductivity with exposure to Cl-2 gas. The sensing properties of the MgPc thin film gas sensors were strongly dependent on the preparation conditions of MgPc thin films. The highest sensitivity for Cl, gas was obtained in the gas sensor using MgPc thin film prepared under the optimized preparation conditions; substrate temperature of 180 degrees C, evaporation temperature of 475 degrees C and film thickness of 40 nm. It was obtained that at operating temperature of 200 degrees C, the sensitivity of the MgPc thin film gas sensor linearly increased with the Cl-2 gas concentration in the range of 0.18-35 ppm. (c) 2004 Elsevier B.V. All rights reserved.