The electrical conductivities of the molten binary salt mixtures Li2CO3 - X2CO3 ( X: Na, K, Rb, and Cs ) and Na2CO3 - Z(2)CO(3) ( Z: K, Rb, and Cs ) were measured using an ac two-probe technique and electrochemical impedance spectroscopy. The electrical conductivities of the eutectic compositions of (Li0.52Na0.48 )(2)CO3 and (Li0.62K0.38 )(2)CO3 at 923 K were 2.06 and 1.31 S cm(-1), respectively. The conductivity of molten Li2CO3 - X2CO3 ( X: Na, K, Rb, and Cs ) and Na2CO3 - Z(2)CO(3) ( Z: K, Rb, and Cs ) decreases with an increase in the ionic radius of X or Z. We have succeeded in correlating the equivalent conductivity of a binary carbonate mixture Lambda, with the composition (x(A) and x(B)), the equivalent conductivities of the pure components A and B (Lambda(A) and Lambda(B)), and two parameters (lambda(A) and lambda(B) ) as Lambda = x(A)(2)Lambda(A) + x(B)(2)Lambda(B) + xAxB(lambda(A)x(A) + lambda(B)x(B)). The pair of interaction parameters (lambda(A) and lambda(B)) can be estimated for a binary system of molten carbonates using the relationship between the cationic radii and the equivalent conductivities of the pure carbonates. (c) 2007 The Electrochemical Society.