Electrical conduction in the monoclinic lanthanum disilicate La2Si2O7 was investigated in H2O/O-2 (D2O/O-2) and H2O/H-2 (D2O/D-2) atmospheres at 500-925 degreesC. It was found that La2Si2O7 became to conduct protons under wet conditions by partially substituting Sr for La. From the HID isotope effect and the partial pressure dependence of the conductivity, it was demonstrated that the protonic conduction in the Sr-doped La2Si2O7 was considerably predominant both in wet oxidizing and in wet reducing atmospheres and that the contribution of electronic conduction to the total conductivity was minor. The conductivities of the 1 mol% Sr-doped La2Si2O7 were 8 x 10(-7)-2 x 10(-5) S cm(-1) under moisturized conditions at 550-925 degreesC, and were relatively lower than the conductivities of the acceptor-doped LaPO4 and LaBO3. The partial pressure dependencies of the conductivity of the Sr-doped La2Si2O7, however, seemed to essentially resemble to those of the acceptor-doped LaPO4 and LaBO3, indicating the similar proton dissolution and conduction mechanism in these oxoacid salts. (C) 2004 Elsevier B.V. All rights reserved.