The microstructures of as-cast and heat-treated ingots and melt-spun and heat-treated ribbons of Nb-17at%Al-xat%Mo alloys (x = 20, 30, 40) were studied. The results show that Mo inhibits the A2 → B2 ordering transformation of the β phase and destabilises ω formation. A ternary ω * phase has been observed for the first time in the B2 matrix of the melt-spun ribbons after heat-treatment at 973 K for 500 h. It is suggested that the B2 → ω * decomposition precedes the precipitation of the equilibrium A15 phase in the B2 matrix, since the ω * phase has a lower nucleation barrier than the A15 structure.