The microstructures of as-cast and heat-treated ingots and melt-spun and heat-treated ribbons of Nb–17 at.% Al have been studied. The results show that Al not only enhances the A2→B2 ordering transition of the β phase, but also promotes formation of the ω phase in B2. A binary o phase has been observed for the first time in the B2 matrix of the melt-spun ribbons after prolonged heat treatment at 973 K. It is suggested that the B2→o phase decomposition precedes the precipitation of the equilibrium A15 phase in the B2 matrix, since the o phase has a lower nucleation barrier than the A15 structure.