The fluid system H2O-NaCl has been subject of investigation for many years in chemistry and physics, and is of special interest to fluid inclusion studies. Both components occur in the majority of fluid inclusions known to geologists. The properties of this fluid system are of major importance to interpret trapping conditions of fluid inclusions in many types of rock. The principle objective of the present study was to investigate experimental re-equilibration behaviour of synthetic H2O-NaCl-rich fluid inclusions. The study was orientated towards detecting diffusion processes, brittle deformation processes (micro-cracking), and fluid inclusion wall re-crystallization in and around synthetic fluid inclusions with well-defined composition and density under well-defined temperature-pressure-fugacity conditions. In due course of the project, a number of additional objectives were defined due to unexpected results: 1. Do synthetic H2O-NaCl fluid inclusions have densities that corresponds to the experimental T-P conditions?; and 2. How can I change fluid inclusion compositions against the applied gradients in fugacity compared to an external fluid? The synthesis of pure H2O and H2O-NaCl fluid inclusions was performed according to the descriptions in the PhD thesis of Gerald Doppler. Fluid inclusions were synthesized at 600 ˚C and 337 MPa, within the α-quartz stability field. These experimental conditions were monitored accurately with an internal thermocouple. Fluid properties were calculated with software to handle complex but accurate equations of state, which are available at the web-site http://fluids.unileoben.ac.at. The re-equilibration experiments were designed at constant temperature (600 ˚C) and constant pressure (337 MPa) and specific fugacity gradients of H2O. The modifications of H2O-NaCl inclusion that are re-equilibrated in pure H2O environment are significant, both Tm's and Th's are strongly affected. The intensity of modification is directly related to experimental run time, which may imply diffusion processes. All the experiments reveal a relative loss of H2O in individual inclusions. Close examination of the outcome of these experiments reveals a number of enigmas. First, fugacity gradients must result in diffusion of H2O into the inclusions resulting in a relative decrease of salinity, that correspond to higher Tm's. Second, Tm's decrease to values below the eutectic temperature of the binary H2O-NaCl system. Third, short term experiments have a reversed modification of Th compared to long term experiments. Fifth, theoretical modelling of H2O diffusion out of the inclusions (calculated with the programs AqSo DH and ReqDif, http://fluids.unileoben.ac.at) result in a completely different trend of paired Th and Tm modifications. The observed modifications in microthermometrical data can only be obtained by preferential H2O loss. However, bulk H2O diffusion alone cannot account for these modifications, but a paired process of H2O loss and SiO2 gain result in observed trend of Th and Tm. This process is not driven by H2O-fugacity gradients and only occurs with H2O-NaCl bearing fluid inclusions. Any pressure difference between fluid inclusions and pore fluids is inhibiting bulk-diffusion processes of fluid components through quartz. This work is supported by the FWF, project P224460-N21, as part of the PhD thesis of Gerald Doppler.