The isothermal metallurgical reaction in two-segmented Cu-Sn nanowires results in the formation of a Sn/Cu6Sn5/Cu3Sn/Cu sandwich structure. In-situ transmission electron microscopy is used to study how Cu6Sn5/Sn and Cu/Cu3Sn interfaces propagate and change shape during the intermetallic compound growth. The Cu6Sn5/Sn interface is observed to evolve from an inclined configuration to a vertical, edge-on configuration with the propagation of the Cu6Sn5 phase towards the Sn segment. The Cu/Cu3Sn interface also becomes less inclined as it propagates toward the Cu segment. This interface evolution is driven by the minimization of the interface energy associated with minimizing the interface area associated with the edge-on interface. The Kirkendall void growth induces the breakage of the Cu segment and results in the Cu3Sn → Cu6Sn5 transformation with the final Sn/Cu6Sn5/void/Cu sandwich structure.