Hydrogen isotope investigations on hydrothermal quartz reveal two H-reservoirs: (i) structurally bond molecular water in homogenously distributed small clusters or bubbles, and (ii) mechanically trapped fluid inclusions. Varying mixing ratios of the two reservoirs are sampled by means of mechanical and thermal decrepitation applied to different grain size fractions. A two component mixing calculation results in an isotopic characterisation of the endmembers which fractionate hydrogen isotopes close to the known MOH-H2O systematic with water being enriched in D. Temperature controls both the internal fractionation as well as the abundance ratio of inclusion water to bonded water. At high temperatures fractionation is small but bonded water becomes more abundant and comprise a significant amount to water from both thermal and mechnical extraction techniques. Hence, the isotope composition of the extracted water does not reflect the original hydrogen isotope composition of the hydrothermal fluid especially at temperatures higher than 200°C. In the literature reported dD data of fluid inclusion which in most cases were used to elucidate the origin of the hydrothermal fluid tend to be too low.