Adsorbate-induced surface resistivity -- the change in electrical resistivity of a metal film when a gas adsorbs on its surface -- provides a simple but powerful probe of the dynamical interaction between conduction electrons and adsorbates.@footnote 1@ The dominant mechanism is generally diffuse scattering of the electrons from the adsorbate, and the scattering cross section per adsorbate is usually nearly independent of adsorbate coverage. For sulfur on Cu(100), however, the resistivity increases rapidly with coverage up to a threshold, and then remains constant as the coverage increases further. This effect was first observed by Xu and Hirschmugl using infrared reflectance and a single crystal sample.@footnote 2@ They attributed the rapid increase at low coverage to a large scattering cross section for S adsorbed on defect sites, with a near-zero cross section for S on terrace sites. In the present study the same behavior is seen in both the dc resistivity and the infrared reflectance of epitaxial Cu(100) thin films. The transition between the two scattering regimes, however, occurs at a higher coverage (~0.2 ML) than in the single-crystal measurements (0.04 ML). The role of defects is investigated by quantitatively estimating the defect density from CO adsorption measurements, and by sputtering the surface before adsorption to increase the defect density. @FootnoteText@ @footnote 1@R.G. Tobin, Surf. Sci., in press.@footnote 2@X.F. Xu and C.J. Hirschmugl, Surf. Sci. 490 (2001) 69.