Si and Si-based alloys are promising candidates for use as negative electrode materials in Li-ion batteries because of their large specific capacities of up to ten times that of graphite. The phases that form during the insertion of Li in the electrode material can have a significant impact on the capacity retention of the material. A series of studies have been performed on the reaction of Li with Si1-xMx films over wide composition ranges, where M is an element that itself alloys with Li. This is the first in a series of three papers that will present the results of these studies for a variety of elements, M. This paper presents the results obtained for the reaction of Li with Si1-xAgx films along with a simple model of the structure changes in the films during the insertion and removal of Li. (c) 2005 The Electrochemical Society. All rights reserved.