The hydration structure of the supersaturated MgSO₄ solutions has been investigated by means of X-ray diffraction at 298 K. An empirical golden rule, r=0.618λ/sin θ, has been used to extract some structural information from the experimental diffraction curve, and the other, r=0.618×4π/s, to expound some peaks on the structural function curve. The calculation of the structural model shows there exist the water-shared ion-pair and the contact ion-pair in the supersaturated solutions. The characteristic Mg-S distance of the contact ion-pair has been determined to be about 0.328 nm. It shows that the contaction-pair MgSO₄⁰ is a unidentate complex. The Mg-O_S(1) distance between magnesium ion and the nearest oxygen atom in sulfate ion has been measured to be 0.205 nm and is equivalent to that of Mg-H₂O. The Mg-O_S(2) distance is close to that of Mg-O_S(3), about 0.355 nm. The Mg-O_S(4) distance is situated at about 0.456 nm. The number of ion-pairs obviously increases with the increasing of concentration. The marked Mg-S distance of the water-shared ion-pair coexisting with the contact ion-pair was refined to be 0.492 nm. The symmetry of the hydration structure of sulfate ion is lowered by complex formation of magnesium ion with sulfate anion.

Key words: liquid structure, supersaturated solution, MgSO₄, X-ray diffraction