As lithiated–metal alloys such as Li–Si or Li–Sn are of great interest as high energy density anodes for Li-ion rechargeable batteries, a fundamental understanding on how the metals behave upon lithiation is important. X-ray diffraction and HREM experiments in this work reveal that the crystallization of equilibrium inter-metallic compounds (e.g. Li–Si) is inhibited during lithiation at room temperature, and that formation of highly lithiated glass instead occurs. This glass is shown to be metastable with respect to the equilibrium crystalline phases. We show that the mechanism of electrochemical alloying is electrochemically-driven solid-state amorphization (ESA), a process closely analogous to the diffusive solid-state amorphization (SSA) of thin films. Experimental results on the diffusive reaction of Li and Si bilayer films support the proposed mechanism.