Giant magnetoresistance (GMR) is reported in Ag1−xCox(x=0.26–0.53) films cosputtered on Si at 28–354 °C from separate Ag and Co targets. GMR ratios (10 kOe maximum field) as high as 0.55 and 0.24 at 5 and 295 K, respectively, are achieved for the Ag0.67Co0.33films deposited at a substrate temperature,Ts, of 125 °C. The GMR ratio increases slightly when theTsincreases from 28 to &bartil;125 °C, and decreases substantially atTs≳175 °C. The resistivity of these films decreases monotonically with increasingTsand levels off to a value &bartil;3× of Ag atTs≥250 °C. The films deposited at lowerTsconsist of essentially a metastable Ag‐Co solid solution, while those deposited at higherTsconsist of separate fcc Ag and fcc Co. The coercivity of these films increases rapidly atTs≫125 °C, consistent with the growth of single‐domain ferromagnetic Co particles in thisTsrange. TheTsdependence of GMR ratios is interpreted based on spin‐dependent interfacial scattering arising from theTsdependence of the size and number of ferromagnetic Co particles within the electron mean free path.