In this thesis the composition and geometric structure of barium titanate - derived ultrathin films, specially oxide quasicrystals (OQC) are determined. After an introduction about the aims of the work, the investigated systems, BaTiO3, SrTiO3, Pt(111), and OQC, are presented, followed by explaining the employed techniques with an emphasis on extended x-ray absorption fine structure (EXAFS). For the analysis of EXAFS modulations a new projection method is developed, which yields more accurate initial values for bond lengths than the Fourier transformation. Employing EXAFS on a BTO-derived ultrathin film, the Ti-O and the Ti-Pt bond lengths have experimentally been determined for the first time. In addition, growth and decay of the BTO-derived OQC have been monitored using X-ray photoelectron spectroscopy. The changes in the spectra show that simultaneously to the rewetting of the Pt surface Ti is reduced from 4+ to 3+ and the number of coordinating O atoms decreases.