Sr and Ba beta-aluminas in a single phase were synthesized in the MO-MgO-Al2O3 (M = Sr and Ba) system by a solid state sintering method. The electrical conductivity was measured at 873 to 1473 K by an AC impedance method. The total electrical conductivities of Sr beta-alumina and Ba beta-alumina were 1.2 x 10(-2) S m(-1) and 6.7 x 10(-4) S m(-1) at 1473 K, respectively. A CO2 gas sensor using the srp-alumina was constructed. The sensor could be used at more than 1000 K.