In order to develop mixed protonic and electronic conductors, we proposed a novel concept for material design that enables to control partial conductivities by fabricating solid solutions of protonic and electronic conductors. In this work, Sr-doped LaBO(3) and Sr-doped CeBO(3) were chosen as model compounds conducting protons and electron holes, respectively. Solid solutions of the above borates, Sr-doped La(1-x)Ce(x)BO(3), were prepared, and their electrical conductivities were investigated in 8.5 x 10(2)-4.2 x 10(3) Pa of p(H(2)O) and 1.0 x 10-1.0 x 10(5) Pa of p(H(2)) at 1073 K. From the experimental results of the gas partial pressure dependences of the conductivities, major charge carrier species were identified as a function of x. It was found that proton was the major charge carrier when x<0.2 while the contribution of the electron hole conduction became remarkable as x increased above 0.2. The contribution of the electron hole conduction can be interpreted by the percolation model. (C) 2010 Elsevier B.V. All rights reserved,