We present the results of a molecular dynamics simulation study of the effects of counterion type on the properties of the interior region of aqueous reverse micelles. The model systems, which treat only the interior region at the atomistic level, are designed to represent water-in-oil microemulsions formed by the aerosol-OT surfactant, either with its usual counterion, Na + , or with the K + counterion. Our study covers the water content, w0 ) [H2O]/[surfactant], range of 1-7.5, where the reverse micelles are approximately spherical and contain tens to hundreds of water molecules in their interior pool. We find that several key structural and dynamical features of the reverse micelle water pool are strongly affected by counterion type. These effects can be ascribed to the differences in headgroup-counterion coordination and to the stronger affinity for water that the smaller Na + ion exhibits. At low water content, K + ions are able to coordinate four headgroups, while Na + coordinates a maximum of three. As w0 increases, this coordination number decreases for both ions but always remains higher for K + , which also has a stronger tendency to remain in the interfacial region than does Na + . K + displaces water from the interface to a larger extent than does Na + , with the result that fewer water molecules are trapped between the headgroups. That and the fact that K + has a weaker attraction for water than does Na + lead to higher mobility of water throughout the reverse micelle interior and more bulklike structural features, such as the number of water-water hydrogen bonds, in the interfacial region.