In an anion exchange membrane (AEM), the dissolution of carbon dioxide (CO2) in water results in the conversion of hydroxide to the less mobile (bi)carbonates. Theory is developed to explain the temperature and CO2 content dependencies of the measured AEM ionic conductivity in terms of distribution and motilities of the mobile ions within the membrane. Such understanding is valuable for developing AEM technologies, such as in (i) deriving properties not easily measurable including pH values, ion concentrations and transport numbers, and (ii) identifying material properties and characterization methodologies that impact the membrane's function and stability in fuel cell applications. (C) 2011 The Electrochemical Society. [DOI:10.1149/2.011112esl] All rights reserved.