Characterization of defects and their sources is essential for developing mitigation solutions to support the production of defect-free extreme ultraviolet (EUV) mask blanks. Enhancements to cleaning processes and the deposition tool are proving to decrease the defect density on mask blanks, and the resulting defect trends can be tracked to determine the effectiveness of these improvements. While standard defect characterization methods such as atomic force microscopy (AFM) and scanning electron microscopy (SEM) can provide useful information for large defects, sub-100 nm defects pose challenges to the current conventional metrology techniques. To address the study of these nanoscale defects, SEMATECH's Mask Blank Development Center (MBDC) houses advanced metrology capabilities that include Auger electron spectroscopy (AES) and high resolution transmission electron microscopy (TEM). Both techniques are providing enhanced compositional analysis capabilities for defect reduction efforts. TEM is proving to be a valuable technique for defect mitigation and is currently supporting many other projects including substrate smoothing activities, deposition simulation development, and defect printability studies. The rising issues with the metrology of increasingly small EUV mask blank defects will be outlined, and comprehensive characterization results using TEM and AES on EUV mask blank defects will be presented.