The effects of additions of Fe and Zn on the relative stabilities of the cubic Ll2 and tetragonal DO22 structures in TiAl3- and NbAl3-base alloys are evaluated using ab initio electronic band calculations. The Fe or Zn distribution on the aluminum sublattice is modeled by a periodic array which corresponds to alloying additions of 12.5 or 25 at.% Fe or Zn. Addition of 12.5 at.% Fe is sufficient to stabilize the Ll2 structure in both TiAl3- and NbAl3-base alloys. Interpolation of the structural energy differences suggests that the DO22 and Ll2 structures have the same energy at —4.5 at.% Fe in Ti(Al, Fe)3, in agreement with experiment,1’2 and at —11 at.% Fe in Nb(Al, Fe)3. The Ll2 stabilization effect per atom of Zn in Nb(Al, Zn)3 is approximately half as large as that of Fe. The stable structure is the one for which the Fermi energy lies in a minimum in the density-of-states (DOS) distribution, as in the binary compounds. The results are explained on the basis of a simple model involving charge transfer to alloying additions on the Al sublattice.