Grand Canonical Monte Carlo Simulation if Water Positions in Crystal Hydrates Abstract: Cavity Biased-Grand Canonical Monte Carlo (CB-GCMC) technique is used to determine the water molecule locations in the orthorhombic form of the sodium hyaluronate crystal hydrate. Even though the agreement with the experiment is good for the most part, the prediction for one of the water molecules is quite different, and our study reveals an alternative possible location for that water molecule. Our results show that, if it is incorporated into crystal refinement studies, Grand Canonical Monte Carlo can be an extremely powerful supplementary method to predict the location and the orientation of the hydrating waters.