A UHV system for in-situ preparation and analysis of ultra thin films has been built. The system includes a rapid thermal processing furnace which allows production of samples over a wide range of temperatures and pressures using isotopically enriched gases. XPS, AES, and LEED analyses provide information on the surface structure and composition. With a transportable UHV chamber, the samples can be transferred to a 4π γ-ray spectrometer facility (in UHV), where analytical ion beam methods can be used to determine isotopic depth profiles and total amounts of isotopes in the films. Furthermore, an ion beam deposition facility (in UHV) can produce isotopically enriched silicon films on Si substrates for in situ isotopic tracing.