Newly developed semiconductor thin-film gas sensors with a high sensitivity for Cl-2 gas using (Zn2In2O5)(x)-(MgIn2O4)(1-x) multicomponent transparent conducting oxide thin films are described. The sensitivity of (Zn2In2O5)(x)-(MgIn2O4)(1-x) thin-film gas sensors could be controlled by altering the composition. The highest sensitivity was obtained using (Zn2In2O5)(0.6)-(MgIn2O4)(0.4) thin films prepared by magnetron sputtering; when operated at 300 degrees C, they were able to detect Cl-2 gas at a minimum concentration of 0.01 ppm. The resistivity, carrier concentration and Hall mobility of the thin-film sensors were measured under operating conditions using the van der Pauw method. An increase or decrease of resistivity in thin-film sensors resulted from an decrease or increase, respectively, of both carrier concentration and Hall mobility. Our experiments showed that carrier transport in thin-film gas sensors is dominated by grain boundary scattering. The adsorption of chlorine results in a free electron trap, the same as that produced by adsorption of oxygen. The sensitivity and the resistance of (Zn2In2O5)(0.6)-(MgIn2O4)(0.4) multicomponent oxide thin-film gas sensors exhibited very stable long term operation in air containing high concentrations of Cl-2 gas.