Structural transitions during aluminum leaching of the NiAl3 phase in a Raney nickel-aluminum alloy have been investigated by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and X-ray energy dispersive spectroscopy. We observed that NiAl3 grains cracked into crystalline nano-fragments at the initial stage of leaching. A possible mechanism for the grain fragmentation was proposed based on the crystal structure of NiAl3. We discovered that fcc nickel, the known active phase, coexisted with another active nickel phase with an orthorhombic structure in the Raney-Ni catalyst. The orthorhombic nickel phase was generated directly from its source phase, NiAl3, and further transformed to the fcc nickel phase during aluminum leaching