In situ X-ray reflectivity measurements have been employed to study counterion overlayers at the interface between electrolyte solutions and monolayers of carboxylic acid terminated alkanethiols self-assembled on Au(lll). The counterion densities at the interface are obtained by fitting the reflectivity data. We found that the density of Cd2+ depends strongly on the pH of the electrolyte solution. At high (>6) pH, dense overlayers, with approximately one cadmium per carboxylic acid group, were observed. At pH > 8, multilayers of Cd(OH)(2) start to grow. The isotherm in a large pH range can be successfully fitted by a modified BET multilayer adsorption model. Investigation of a variety of other cations (Pb2+, Ba2+ Ca2+, and Cs+) indicates that strong chemical interactions between the carboxylate groups and counterions are more important than purely electrostatic interactions in inducing dense counterion overlayers. The assembled hydrocarbon chains were found to preserve their ordered root 3 x root 3R30 degrees structure in the electrolyte solutions, though no indication of in-plane order in the counterion overlayer was obtained.