Novel metal oxide films and new metal gates are currently being developed for future generations of Si based field-effect transistors as the SiO 2 gate dielectric and polycrystalline Si gate electrode are reaching scaling limits. These new gate stacks are often comprised of subnanometer layers. Device properties are increasingly controlled by the complex structure and chemistry of interfaces between the layers. Electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) is capable of providing insights into interfacial chemistry and local atomic structure with a spatial resolution unmatched by any other technique. Using gate stacks with Hf-silicate dielectrics as examples, we demonstrate the capabilities of STEM/EELS for analyzing the interfacial chemistry of novel gate stacks. We show that a-priori unknown reaction layers of a few Å thickness can be detected and identified even in the presence of substantial interfacial roughness that may obscure such layers in a highresolution image. We discuss some experimental aspects of STEM/EELS chemical profiling applied to gate stacks and the factors affecting the interpretation. In particular, the effects of interfacial roughness, beam spreading, elemental analysis in a heavily scattering matrix, and the interpretation of the EELS core-loss fine-structures from ultrathin layers are discussed.