We have measured the electrical power density of a single cell of a solid oxide fuel cell (SOFC) with (Ba0.3Sr0.2La0.5)InO2.75 as an electrolyte at an operating temperature of 800 degrees C. A moistened mixture of 10% hydrogen in argon, constituted the fuel gas; air was the oxidant gas. We selected (La0.6Sr0.4)(Mn1-x)O3-delta (M=Co, Ni) (x=0-0.3) and Ni as cathode and anode materials. The power density was dependent on the amount of the transition metal doped into the cathode; the use of (La0.6Sr0.4)(Mn0.9Co0.1)O3-delta as a cathode material enabled us to reach a 2 maximum power density of 0.61 W/cm. We also investigated the diffusion of the constituent elements around the interface between the cathode and the electrolyte by X-ray diffraction and X-ray energy dispersive analysis (EDX). We found that the constituent elements Mn, Co, Ni and In diffused to the opposite phase. We suggest that this cation diffusion would change the electrical conductivity of the cathode and electrolyte around the interface and would affect the cathodic reaction. The interface resistance between the (Ba0.3Sr0.2La0.5)InO2.75 electrolyte and the (La0.6Sr0.4) (Mn1-xMx)O3-delta (M=Co and Ni) (x=0-0.3) cathode was characterized by the direct current two-probe method. It showed minimum values at x=0.1(Co) and x=0.2(Ni), suggesting that the interface properties might strongly affect the power generation of the SOFC. (c) 2006 Elsevier B.V. All rights reserved.