Abstract　 　　 

　The composition of a planet and its atmosphere are governed by its formation history in the circumstellar disk and its further evolution. During the early formation phase a rocky core is formed, which may subsequently accrete a gaseous envelope once the core mass succeeds a critical value, forming a giant planet. While Hydrogen and Helium dominate the mass of the envelope and atmosphere of giant planets, their spectral appearance is dominated by the abundance of heavy elements needed to build spectroscopically active molecules (e.g. water, methane). The molecular composition of mature exoplanet atmospheres depends on the relative abundances of heavy elements in the envelope, which follow from the accretion history during the formation phase. In this talk we will present model calculations of the formation of giant planets, following which kind of material is accreted depending on the formation location in the disk. We will focus on the circumstances under which an oxygenrich or a carbonrich chemistry results. We also show the resulting atmospheric spectra of mature planets, and simulated observations of transiting systems with which the discussed cases can be distinguished.