Using an (indirect) analytical technique based on the measurement of the mother liquor composition in the liquid water chlorine gas - solid chlorine hydrate system, a new method has been developed for the determination of the composition of chlorine hydrate. This method eliminates the errors caused by the decomposition of hydrate and the inclusion of mother liquor. Using this method, the real composition of chlorine hydrate can be determined with accuracy of +/-0.1 H2O/Cl-2. Furthermore, if solid solution theory is applied, the space occupation at a given composition of chlorine hydrate can also be calculated.