A series of thin film Ag1-xSix alloys were fabricated using radio frequency sputter deposition with composite Ag+Si sputter targets and water-cooled and liquid nitrogen-cooled substrate holders. The microstructures of the films were analyzed using x-ray diffraction and transmission electron microscopy. As-deposited films contained fee Ag and amorphous Si. The Ag grain size decreased as the Si content increased. Films deposited on the liquid nitrogen-cooled substrate holder exhibited grain sizes smaller than films deposited on the water-cooled substrate holder. Despite the negligible Ag and Si mutual solid solubilities, the metastable Ag3Si hcp silver silicide phase was observed in addition to fee Ag and amorphous Si in a 12 at. % Si alloy deposited on the liquid nitrogen-cooled substrate holder. The Ag3Si phase observed by x-ray diffraction in 5000-Angstrom-thick films was not found in the 250-Angstrom-thick TEM films, suggesting a thickness dependence of stability. During vacuum anneals above 100 degrees C, the Ag3Si phase partially decomposed.