To develop an understanding of the reactivity and selectivity for methanol oxidation on the sulfur-modified Fe(100) surface, a simulation study of adsorbate island morphologies was carried out. Adsorbate islands having compact and dendritic morphologies were prepared for various coverages of a c(2 x 2) overlayer on a square lattice. The surface-site types were determined and the site populations were monitored as functions of adsorbate coverage. The effects of island number and size were determined. On the basis of a perimeter active-site hypothesis, the site-population distributions were compared to the reactivity profiles for oxidation of methanol to CO and formaldehyde as a function of sulfur coverage on the Fe(100) surface.