Utilizing the splat-cooling technique, Nb-Ni alloys with 33–78 at.% Ni and Ta-Ni alloys with 55–67 at.% Ni were rapidly quenched from the melt. The thin foils or flakes obtained were investigated by X-ray and electron diffraction. The diffraction patterns exhibited only broad maxima instead of relatively sharp crystalline peaks. Computer calculations based on several microcrystalline models were able to account for the features of the diffraction pattern in the region of the first maximum; best agreement was attained by assuming microcrystals of about 150 atoms in a body-centered-cubic W-A2 type configuration (equivalent to spherical microcrystals 15–16 Å dia.). The microcrystalline structure persists for 1 hr at 600°C; at higher temperatures it decomposes into the equilibrium phases. In the Nb-Ni decomposition, a crystalline transition phase is first formed. The transformation of the microcrystalline Ta-Ni phase to TaNi 2 is regarded as an ordering reaction, combined with particle growth. A crystal-chemical relationship of the new microcrystalline phases to α-Fe may exist.