The structure and electrochemistry in lithium cells of sputtered thin films and ball milled alloys in the Cu-Si system (0 <= x <= 0.60 in CuxSi1-x) were investigated. Both thin films and ball milled Cu-Si alloys showed the co-existence of amorphous Si and a nanocrystalline Cu3Si phase. The lithiation potential of Cu-Si alloys was found to be significantly shifted to potentials below that of pure Si. In addition, the Cu3Si phase was found to be involved in the electrochemical reaction of the alloys with lithium, however not all of the Si could be reacted with Li to its full theoretical extent. This suggests that a ternary Cu-Li-Si phase may be formed at full lithiation of these alloys. (C) The Author(s) 2016. Published by ECS. All rights reserved.