We demonstrate the utility of electrospray gas phase ion-mobility analysis as a new method to investigate nanoparticle flocculation, or aggregation. Au nanoparticle (Au-NP) solutions were sampled via electrospray (ES), followed by differential ion-mobility analysis (DMA) to determine the particle mobility distribution. Multimodal size distributions obtained with ES-DMA indicated the presence of single Au-NPs (monomer) as well as larger Au-NP clusters such as dimers, trimers, and tetramers under specific solution conditions. The fraction of each aggregate species as a function of time was quantitatively characterized from which the degree of aggregation, aggregation rate, and stability ratio at different ionic strengths was determined. The latter enabled extraction of a surface potential (or surface charge density) of 64 ± 2 mV for 10 nm Au-NPs, which is in good agreement with values obtained from other methods, thus validating our approach. Our results show that ES-DMA is a valuable tool to quantitatively probe the early stages of colloidal aggregation or as a preparatory tool for size election of aggregates.