An amperometric sensor for the measurement of D2O concentration in a mixture of H2O and D2O has been studied using CaZr0.9In0.1O3-alpha as a high temperature protonic conductor. When the sensor heated at 1173 K was exposed to air passed through the H2O + D2O mixture at 293 K, it could respond to D2O in the concentration range of 5 to 100% with a 90% response time of about 15 min.