The effects of isovalent cation substitution in yttrium-doped barium zirconate on microstructure and bulk conductivity were examined. Substitution of titanium ion for zirconium ion and substitution of strontium ion for barium ion in yttrium-doped barium zirconate reduce the bulk conductivity and promote grain growth. These two substitutions are expected to increase the concentration of yttrium in A site (barium site). Well-grown grains are indirect evidence of the increase in the concentration of the dopant cation into A site because it has been reported that extra barium oxide promotes grain growth. The titanium substituted yttrium-doped barium zirconate showed the higher activation energy and lower pre-exponential term for proton jump than those of yttrium-doped barium zirconate. Strontium substituted yttrium-doped barium zirconate showed the higher activation energy but the higher pre-exponential terms. (C) 2009 Elsevier B.V. All rights reserved.