The biopharmaceutical industry characterizes and quantifies aggregation of protein therapeutics using multiple analytical techniques to cross-validate results. Here, we demonstrate the use of electrospray-differential mobility analysis (ES-DMA), a gas-phase and atmospheric pressure ion-mobility method for characterizing protein aggregates. Two immunoglobulin Gs are systematically heat treated to induce aggregation and characterized using size-exclusion chromatography (SEC) and ES-DMA. Although ES-DMA is a gas-phase characterization method, we find that aggregation kinetic rate constants determined by ES-DMA is in good agreement with those determined by SEC. ES-DMA appears to have a higher resolution and lower limit of detection as compared with SEC. Thus, ES-DMA can potentially become an important orthogonal tool for characterization of nascent protein aggregates in the biopharmaceutical industry.