Under conditions typically found in the Earth’s crust, there is a large pressure-temperature-composition rangein the H2O-NaCl system where fluids may separate into a low-salinity vapor end member and a high-salinityliquid end member. However, heterogeneous trapping is common during the formation of fluid inclusions in animmiscible fluid system, violating the fundamental assumption of homogeneous entrapment for fluid inclusionmicrothermometry. This has profound consequences on the composition of these fluid phases and consequentlyon the formation of ore deposits from hydrothermal systems. At the same time, minor mixture of the highsalinity liquid phase with the low-salinity vapor phase cannot be distinguished from the end-member vapor-richfluid inclusions by their bubble sizes. Precise determination of the salinities of vapor-rich fluid inclusions usingmicrothermometry is deterred by the very small proportion of the liquid phase and the limitation of the analyticalprecision. All this will lead to erroneous compositional data from fluid inclusion analysis.We have quantitively calculated the variations of fluid inclusion properties caused by heterogeneous entrapment during phase separation in the H2O-NaCl system and showed that the salinity, and thereby the elementcontents and element/Na ratios of the vapor-rich fluid inclusions, is significantly changed in heterogeneouslytrapped fluid inclusions. The addition of 1 vol % of the high-salinity end member to the low-salinity end member results in a salinity change from 0.99 to 4.4 wt % NaCl equiv at the phase separation condition of 800 barand 750°C. This will have a significant impact when it comes to determining the concentration of elementstransported as chlorine complexes and typically leads to an overestimation of the mass transport capability bythe vapor phase. Therefore, every effort should be taken during microthermometry to find and measure theleast affected vapor-rich fluid inclusions. Our numerical calculations and synthetic fluid inclusions reveal thatthere is a linear relationship between the salinity and element contents for fluid inclusions at different extents ofheterogeneous entrapment. Therefore, linear correction to the measured lowest vapor salinity can obtain a goodapproximation of the element contents in the end-member vapor phase.