When epitaxial Cu(100) films are exposed to formic acid, oxygen or CO gas at low temperature, both the de electrical resistance and the infrared reflectance of the film change. The ratio of the resistance change to the reflectance change, however, is approximately twice as large for formic acid and CO as for oxygen, on identically prepared samples. This difference in slope is inconsistent with models in which the increase in film resistance arises solely from scattering of conduction electrons by the adsorbates. Changes in conduction electron density may also be important for some systems, especially when the chemisorption bond involves large charge transfer.