The aggregation of starburst PAMAM (poly(amidoamine)) dendrimers (bearing either 64 carboxylic or amine peripheral groups) from micromolar aqueous neutral solutions on the surface of bare and thiol (HS(CH2)(2)COOH or HS(CH2)(2)NH3Cl) modified gold electrodes has been studied using the QCM technique. As expected, the electrostatic interaction between protonated amine groups and carboxylate functional groups has been shown to promote much larger surface coverage values than those obtained for naked clean gold substrates. Computer fitting of the obtained titration curves to Langmuir and Frumkin isotherm models suggests that the main effect of the surface attached thiol is not an increased overall adsorption energy but a favorable lateral interaction between the adsorbed dendrimer molecules.