The translational symmetries of two structurally complex phases of the AlTa system were determined by means of X-ray and electron diffraction methods. The thermal stability ranges of the phases were delimited and the phase relations in the medium range of the AlTa system were unravelled: Al u Ta ( u ≈ 1.5), triclinic, aP ∗, a = 1074.8(3) pm, b = 1107.8(4) pm, c = 1043.3(4) pm, α = 90.45(2)°, β = 97.60(2)° and γ = 63.19(2)°, is stable below about 1500 K and decomposes in the solid state to yield entropically stabilized Al 69Ta 39 (cF432) and Al v Ta ( ν ≈ 1.4), hexagonal, hP ∗, a = 3214(2) pm and c = 1341(1) pm. Al v Ta itself transforms in a peritectoid reaction above about 1620 K into Al 69Ta 39 and Al w Ta ( w ≈ 1.0) of monoclinic symmetry.