Continued scaling of silicon technology requires a paradigm shift in the materials used as gate dielectric in complementary metal-oxide-semiconductor (CMOS) devices. Currently, alternative dielectrics, such as ZrO2, HfO2, Y2O3 and their alloys with SiO2 or Al2O3, are being investigated. High-resolution analytical capabilities afforded by scanning transmission electron microscopy techniques are essential in analyzing interface stability and defects in these ultrathin layers. Z-contrast imaging is used to obtain atomic resolution images that are chemically sensitive and to position the probe for electron energy-loss spectroscopy (EELS). EELS is used to measure composition and bonding across the gate dielectric. The near-edge fine-structure of EELS edges can be used to fingerprint phase formation and nonstoichiometry. Conventional high-resolution transmission electron microscopy is used to investigate crystallization. We show that MBE ZrO2/Si annealed under moderately oxidizing conditions form a low-k interfacial SiO2 layer through oxygen diffusion through the ZrO2 and silicon consumption at the interface. Layers annealed under moderately reducing conditions do not show extensive SiO2, formation, whereas layers annealed under even lower oxygen partial pressures form an interfacial silicide, consistent with thermodynamic estimates. In contrast to ZrO2, CVD grown Y2O3 films show extensive silicate formation upon annealing, through Si diffusion into the dielectric and due to excess oxygen in the films. We show that thin films transform to an amorphous yttrium silicate upon annealing, whereas thicker films form an interfacial silicate and crystalline Y2O3 on the surface. We will discuss possible mechanisms, in particular the role of crystallization, to explain the observed results.