The electrical conductivity of A(3+)B(3+)O(3)-type perovskites, (La0.9Sr0.1)(MO3-delta)-O-III (M-III = Sc, In, and Lu) (LSM), was investigated under controlled atmospheres between 573 and 1273 K. At P(O-2)>10(-5) atm, all the compounds showed the predominant proton conduction below 873 K under wet conditions; at P(O-2)<10(-10) atm, the Sc and In compounds below 873 K and the Lu-one at all temperatures under hydrogen-containing conditions. The Sc compound showed the highest proton conductivity of ca. 4 x 10(-3) S (.) cm(-1) at 873 K in the presence of water vapor or hydrogen. The proton conductivity of LSM decreased with increasing the size of B-site (M-III) cation.