A galvanic cell-type Cl-2 sensor which works at room temperature has been successively constructed with a gastight PbCl2-K2SO4 solid electrolyte, RuO2 as a sensing electrode, and a Pb reference electrode embedded into the electrolyte. By adding Al2O3 powder to the electrolyte, the disk was increased in mechanical strength. The cell arrangement can be represented asPb \ PbCl2 + 1 mol % K2SO4 + 10 mol % Al2O3 \ Cl-2 RuO2The cell showed a reproducible electromotive force (emf) at room temperature in the Cl-2 concentration tested. The measured emf was in close agreement with that given by the Nernst equation for Cl-2 concentrations from 10 to 10(6) ppm in air. The 90% response time of the emf for step changes in the Cl-2 gas was less than 10 s at concentrations higher than 10(4) ppm. The activity of the sensor was maintained without its decay for longer than 6 months.