Mg50Co50 alloybefore and after hydrogenation was investigated by means of transmission electron microscopy (TEM).Mg50Co50 alloy before hydrogenation was found to contain crystals not larger than5 nm in size. Selected-area electron diffraction patterns (SAEDPs) revealedthat these nanocrystals have a body-centered cubic (BCC) structure with alattice parameter of about 0.3 nm. Distribution of Mg and Co elements in theMg50Co50 alloy was uniform, indicated by energy dispersive x-ray spectroscopy(EDS) analysis. Crystallization and decomposition occurred in theMg50Co50 alloy during hydrogenation. A large number of crystals larger than 10 nm were observed inthe hydrogenated sample. The SAEDPs showed polycrystalline rings corresponding to theBCC phase and the Co metal phase. The existence of Mg-rich Mg–Co crystals and Coparticles was also confirmed by TEM–EDS analysis.