In this study, the dissolution of copper nanoparticles in aqueous low-pH suspensions is examined. The dissolution phenomenon is examined using both bulk measurements of copper ion production, as detected by inductively coupled plasma-optical emission spectroscopy (ICP/OES), and a decrease in nanoparticle size using particle-sizing instruments. For size measurements, an electrospray atomizer coupled to a scanning mobility particle sizer (ES-SMPS) was used to monitor changes in the particle size distribution (PSD) of the copper nanoparticles as they dissolved in hydrochloric acid solution in real time. Measured PSDs show interesting changes during the dissolution process, including a change in modality (mono to multi) with time. Although there may be several causes for the observed modality changes upon dissolution, it is clear that only through direct measurements of nanoparticles and nanoparticle PSDs can these dynamic details be captured as these particles change size, thus providing important insights into nanoscale processes.