[en] The sulfur gas potentials in the H/sub 2/S + H/sub 2/ buffer gases were measured by a galvanic cell Ps/sub 2/(g),Au(Pt)/(MoS/sub 2/ + CaS)/CaF/sub 2//(Cu + Cu/sub 2/S + CaS)/Au(Pt) in the temperature range from 650⁰ to 950⁰C and Ps/sub 2/ region from 10/sup -2/ to 10/sup -10/ atm. A quick response time (within 5 to 10 min) in emf with the change of Ps/sub 2/ at a given temperature was observed by placing a MoS/sub 2/ and CaS mixed pellet auxiliary electrode at the bottom of the cylindrical single-crystal CaF/sub 2/ electrolyte. The observed emf's agreed well with with those calculated from the Nernst equation. Using this sensor, Ps/sub 2/ values in the SO/sub 2/ + H/sub 2/ + H/sub 2/S gas system were also evaluated from the measured emf at 827⁰C and were found to be in close agreement with those calculated from the thermochemical tables