With the objective of better understanding the Brust synthesis reaction, this paper examines the evolution of the core sizes of hexanethiolate monolayer-protected Au clusters (MPCs) in a typical synthesis reaction mixture, at time intervals over the course of 125 h. Transmission electron microscopy shows that the average MPC core diameter gradually increases over the first 60 h of reaction and then remains largely unchanged afterward at similar to 3.0 nm. Differential pulse voltammetry of purified MPC aliquots removed from the synthesis reaction exhibit quantized double-layer (QDL) charging peaks. QDL charging peaks have been previously shown to be a strong function of MPC core size and dispersity and reveal (i) the presence of several discernible core sizes in each sample and (ii) an increase in cluster capacitance (C-CLU) with longer reaction times, consistent with the electron microscopy results.