We report on the use of photoemission to probe the electronic structure of ultrathin films. The electronic structure of Zr3N3 grown by low energy N-2(+) implantation in polycrystalline zirconium has been investigated. The insulating characteristics of Zr3N4 as well as the metal-insulator phase transition in ZrNx, when x approaches 1.33, can be nicely observed by photoemission during the implantation process. Other examples correspond to the understanding of oxide/oxide interfaces by in situ characterization of the growth of an oxide film on a dissimilar oxide. We have studied the electronic properties of the TiO2/SiO2 interface by examining the O 2p valence band and the O 2s and Ti 3p core levels during growth of ultrathin films of TiO2 on SiO2. We report evidence for the formation of cross linking oxygen ions to form Si-O-Ti bonds at the interface. In addition we use the high sensitivity of the Ni 2p XPS core level lineshape to the local co-ordination of the nickel atoms to study the interface NiO/MgO(100) by in situ characterization of the growth of ultrathin NiO films an MgO(100).