Abstract The conditions for the formation of evaporated f.c.c., b.c.c. and β-tantalum films have been studied. It has been established that β-tantalum nucleates on clean (UHV “ in situ” cleaved) MgO surfaces; f.c.c. nucleates on surfaces that contain ∼ 1 monolayer of adsorbed impurities. The conversion of f.c.c. to an expanded b.c.c. structure occurs by distortion of a body centered tetragonal cell; Δ v ≊ 0 for the transformation. The stability of the f.c.c. phase is in part due to its very thin film form (< 100 Å) and the large contribution of surface forces, but a necessary factor is the incorporation of surface impurities at a level near a few atom % in a 50 Å film. The formation of epitaxial β-tantalum by slow deposition (0.2 Å/sec) onto in situ cleaved MgO is reported.