When the Pt(100)(root 2 X 5 root 2)R45 degrees-I adlayer is desorbed at negative potentials and then readsorbed at more positive potentials only a 1 X 1 LEED pattern is obtained. Consequently iodine desorption from this surface is not reversible. The same 1 X 1 LEED pattern is obtained if iodine is electrochemically adsorbed on an Ar-cooled (i.e. iodine-free) surface. The iodine packing density on this surface is nearly 30% lower than on the Pt(100 root 2 X 5 root 2)R45 degrees-I adlayer. In contrast to Pt(100) iodine adsorption on a Pt(110) electrode results in a (4 X 1) adlayer under all these adsorption conditions.