Microstructure formation mechanism through competitive reactions during initial hydrogenation in Mg/Cu super-laminate composites (SLCs) was investigated. Mg/Cu SLCs (Mg2Cu composition) were fabricated by accumulative roll bonding (ARB) and composed of laminate structures of Mg and Cu layers with the thickness of few hundreds nm. During the heating process of initial hydrogenation of Mg/Cu SLCs, hydrogenation of Mg and alloying of Mg with Cu followed by hydrogenation of Mg2Cu occurs competitively. It is found that microstructures of Mg/Cu SLCs during initial hydrogenation have changed drastically depending on the order of hydrogenation of Mg and Mg2Cu. The microstructures of Mg/Cu SLCs after initial hydrogenation can be categorized in three types such as (1) MgCu2 network, (2) MgCu2 sheath and (3) MgCu2 layer. Features of differential scanning calorimetry (DSC) profiles of the first cycle were well explained by this microstructure formation mechanism. In order to achieve only MgCu2 network structure, it is important to get fine, even and uniform microstructures in Mg/Cu SLCs. The large number of ARB cycles is inefficient. Changing flow properties such as annealing during ARB, warm-rolling and ultrasonic assisted rolling can be good strategies for that purpose.