Electromagnetic levitation was used to study supercooling effects on solidification characteristics of Al-Nb alloys containing 15-53 at.% Nb. The results indicate that Al3Nb, with a melting temperature of 1980 K, is not a stoichiometric line compound but has a homogeneity range on the Nb side. These results are consistent with the more recently assessed Al-Nb phase diagram, but not with those assessed previously. Significant changes in the microstructural morphology of Al-40 at.% Nb were also observed as a function of the bulk supercooling. These include metastable compositional extensions of Al3Nb, preferential nucleation of Al3Nb in the supercooled liquid and corresponding changes in the solidification path. For example, spherical domains comprising of a mixture of Al3Nb and AlNb2 were revealed in the eutectic microstructure, their volume fraction being an increasing function of the degree of supercooling. Moreover, the Al3Nb and AlNb2 exhibited metastable compositional extensions up to 31 at.% for the former and down to 48 at.% Nb for the latter.