This PhD thesis is structured with six chapters that describe the experimental study on re-equilibration of synthetic fluid inclusions. Three chapters represent autonomous scientific units, which have been published in international scientific journals (one is in review). Another chapter explains the design of the used experimental setup of the hydrothermal laboratory. This thesis is complemented by an overall introduction chapter and by a general conclusion chapter. In literature, some examples have been published about evidence of post-entrapment changes of fluid inclusions. Nevertheless fluid inclusion studies in general are still considered and interpreted with a closed-system behavior. Thus the present experimental work is designed to test potential post-entrapment behavior of fluid inclusions in quartz by systematically performed re-equilibration experiments. Different fluid species are used to determine the major factors of fluid inclusion modifications and their magnitude. Knowledge of diffusion rates of water-related chemical species and isotopes in quartz is important for interpreting fluid-rock interactions based on fluid inclusion research and reconstructing of the P-T-t history of earth’s crustal rocks. As a first approach pure H2O and D2O fluid was used to determine the re-equilibration potential of fluid inclusions; 1) distance to the quartz crystal surface, 2) the size of the inclusions, and 3) the time of re-equilibration experiments on the magnitude of fluid inclusion post-entrapment changes. This series of experiments was published in Contributions to Mineralogy and Petrology 165:1259-1274 (2013). Further investigations were based on the influence of the - quartz phase transition on the post-entrapment fluid inclusions changes was presented in a publication in Lithos 198-199:14-23 (2014). For getting insight in diffusion behavior of NaCl-H2O fluid inclusions in quartz, adequate series of re-equilibration experiments have been performed and submitted to the scientific journal Chemical Geology (2014) which is now under review. The newly acquired experimental data, obtained from this study, support the interpretation of natural fluid inclusions and the inclusions hosting rocks. It provides a framework of conditions which may result in post-entrapment modifications of fluid inclusions at constant PT. More detailed starting information of each sub-project of this thesis precedes as sub-abstract of every chapter.