Molecular Beam Epitaxy was used to grow single-crystal b.c.c. Co/Fe multilayers on GaAs(110BAR). The epitaxial relation between Co, Fe and GaAs has been determined during growth with Reflection High-Energy Electron Diffraction (RHEED). The presence of metastable b.c.c. Co was confirmed by Co-59 NMR experiments. The thickness limit for the stability of the b.c.c phase established from the ex situ NMR study in a sandwiched structure is found to be twice the value determined from the in situ RHEED observed on an open surface. This shows that the Fe epilayer causes recrystallisation of the top atomic layers of Co to the b.c.c. structure.