The structures of magnesium chloride thin films grown on a Pd(111) surface have been studied using low-energy electron diffraction (LEED), temperature-programmed desorption, and Auger electron spectroscopy. At coverages slightly less than 1 monolayer a Pd(111)-(4x4)-MgCl2 LEED pattern is observed by LEED that correlates with a coincident lattice match between Pd(111) and the (0001) or (001) face of either alpha or beta close-packed MgCl2, respectively. At full monolayer coverage, the existence of significant attractive adsorbate-substrate interactions is sufficient to induce a lattice compression of 0.36 Angstrom in the MgCl2 ionic solid in order to maximize the surface coverage. This more compressed structure gives rise to a Pd(111)-(root 13x root 13)R13.9 degrees-MgCl2 overlayer. Upon deposition of thicker films the bulk structure is recovered, producing a simple MgCl2-(1x1) LEED pattern characteristic of the (001) orientation of crystalline MgCl2. This transition is consistent with the greater strength of interactions between successive MgCl2 layers compared to those between the Pd(111) surface and the first MgCl2 monolayer.