Measurements of adsorption-induced changes in the electrical resistivity of thin Cu(100) films show that small amounts of preadsorbed sulfur dramatically reduce the resistivity caused by subsequently adsorbed oxygen. Previous measurements with sulfur and oxygen alone had shown that these two atoms, although chemically very similar, exhibited very different surface resistivity behavior.@footnote 1@ For oxygen the resistivity increases linearly with coverage, indicating minimal interactions between the oxygen atoms. Sulfur exhibits a strongly nonlinear coverage dependence, suggesting that at high coverages interactions between the adsorbed sulfur atoms reduce their cross section for scattering the metal's conduction electrons. In the present experiments the resistivity induced by oxygen was measured on 50 nm-thick Cu(100) films predosed with varying amounts of sulfur. For all sulfur coverages studied the oxygen-induced resistivity varies linearly with oxygen coverage, but the slope of the resistivity vs. oxygen coverage curve is strongly affected by the sulfur. This slope is a measure of the scattering cross section of each adsorbed oxygen atom. A sulfur coverage as low as 0.03 ML reduces the slope by a factor of two from its value on a sulfur-free surface. Evidently adsorbed sulfur atoms suppress the scattering cross section of coadsorbed oxygen even more strongly than they affect the cross section of other sulfur atoms. The saturation oxygen coverage is not significantly reduced at the sulfur coverages studied, indicating that site-blocking by sulfur is not involved. Rather the explanation must lie in changes in oxygen's electronic structure due to chemical interactions with the sulfur. @FootnoteText@@footnote 1@R.G. Tobin, Surf. Sci. 524, 183 (2003).