Redox-based memristive devices (ReRAM) are considered for future energy-efficient non-volatile memory. Memristive cells enable high speed data access, low power consumption and high scalability, which is a key requirement for their use as artificial synapses in neuromorphic computing. Applications in networks of up to 10⁶ devices require a scaling of the cell volume to about 10³ nm³. For a typical valence-change-mechanism (VCM)-type switching cell with a metal oxide, like HfO₂, sandwiched between two metal electrodes of different oxidation enthalpy, this means that the individual layer thickness is in the range of a few nanometers. Therefore, a control of the device properties demands for a control of interfacial reactions during device fabrication. Here, we are going to present a careful analysis of the oxidation of an Hf electrode layer during atomic layer deposition (ALD) of different metal oxides, these are Al₂O₃, TiO₂, and HfO₂. The in-situ encapsulation of a reactive metal electrode by means of an ALD metal oxide layer enables a better process control compared to stack structures formed by breaking the vacuum conditions. In this study, plasma and thermal ALD processes were used to grow ultrathin layers of Al₂O₃, TiO₂, and HfO₂ on top of Hf metal films, after transfer under UHV conditions. The growth of homogeneous metal oxide films of a few nanometer in thickness enabled a systematic analysis of the HfO₂ formation at the Hf/metal oxide interface by means of angle dependent X-ray photoelectron spectroscopy (XPS). The results show that the properties of the interfacial HfO₂ layer can be controlled by the metal oxide deposited on top as well as by the process chemistry and deposition temperature. In contrast to the TiO₂ film, the Al₂O₃ layer clearly serves as oxidation barrier. In addition, the chemistry and structure of the HfO₂ interfacial layer were analyzed from XPS and transmission electron microscopy analysis. Memristive device were fabricated from the Hf/HfO₂/MO_x stacks by depositing Pt top electrodes. The initial current of the devices scales with the pad size indicating the high quality of the insulating oxide layers. Consistent with VCM-type memristive devices the cells required an electroforming voltage to enable bipolar-type resistive switching. In this presentation, the trade-off between the two functionalities of oxidation protection versus enabling of a low electroforming voltage will be discussed.