Metal/oxide interface is a fundamental component in many devices such as magnetic sensors, piezoelectric transducers, and capacitors. The electrical properties of metal/oxide heterostructures are determined by the type of contact formed at the interface, i.e. Schottky or Ohmic contact. As devices are becoming smaller in size, interfaces are becoming even more important due to an increase in the surface to volume ratio. The metal/oxide interface is also a key component in composite multiferroics consisting of ferromagnetic metals and ferroelectric oxides. Recently, in search for multifunctional materials, the composite multiferroics are acquiring a lot of attention. These materials not only have two or more ferroic properties simultaneously (such as ferroelectricity, ferromagnetism) but there may also be a coupling between these ferroic orders, known as magnetoelectric (ME) coupling. Such a coupling allows control of magnetic properties via an electric field and control of electric properties via a magnetic field. Since the coupling originates at the interface between the two components, the composite properties are significantly influenced by interface characteristics. Therefore, in order to realize composite multiferroics with significant ME coupling as well as structurally stable interfaces for electronic device applications, it is important to understand metal/oxide interfaces and to identify the effect of an electric field on the interface structure and properties. The aim of this thesis work is to study the chemistry and magnetic spin structure of FM-metal/FE-oxide (multiferroic composite) interfaces and investigate the effect of an applied electric field on their structural, chemical and magnetic state. For this purpose, we use the unique possibilities offered by isotope sensitive techniques to selectively probe the interface chemistry and magnetism in different types of metal/oxide heterostructures before, during and after the application of an electric field. This is done by enriching the interface with 57Fe which is a Mössbauer active isotope. Two complementary characterization techniques (Mössbauer spectroscopy and nuclear resonant scattering of synchrotron radiation) are employed to investigate different types of metal/oxide interfaces such as Fe/BaTiO3, Fe/LiNbO3, Fe/SrTiO3, Fe/BiFeO3 and Fe/MgO. Based on the results, we propose a model for the electric field–induced modifications of the metal/oxide interface. A correlation between the oxide properties (such as the electrical permittivity, work function, and ferroelectric polarization) and the electric field-induced ion transport across the metal/oxide interface is established. The work function difference between the metal and the oxide determines the initial charge build-up at the interface. The electric permittivity and polarization of the FE-oxide are responsible for the electric field-induced ion transport across the interface. Application of an electric field above a threshold field value results in the formation of a thick intermixed interfacial layer and leads to an irreversible decrease of the magneto-electric coupling properties. Therefore, multiferroic studies on FM/FE heterostructures should be performed at electric fields below the threshold field value. The interface between the metal and a very high dielectric constant oxide can be oxidized or reduced depending on the polarity and magnitude of the applied electric field. The final state of the interface is determined by the polarization history of the heterostructure. These results provide a better understanding of electric field driven ion transport at metal/oxide interfaces and have important implications for the further development of composite multiferroic and complex oxide heterostructures in general.