The microstructure of nanowires of group III-V elements grown under vapour-liquid-solid (VLS) conditions by Au catalysed Molecular Beam Epitaxy (MBE) has been investigated by electron microscopy using High Resolution Transmission Electron Microscopy (HRTEM) and probe corrected Scanning Transmission Electron Microscopy (STEM) in High Angle Annular Dark field (HAADF) mode. Atomic resolution imaging has been achieved from an InAs nanowire terminated by a segment of GaAs. Using the GaAs and InAs lattices respectively on each side of the transition as references, atomic positions and peak intensities have been determined column by column to monitor the composition change and the displacements of the atomic columns across the transition zone. For wurtzite wires with a thickness around 20 nm the reduction in lattice parameter of~7% from InAs to GaAs is accommodated by distortions of the lattice along the sides of the wire and an associated bulging of the lattice planes perpendicular to the wire axis keeping the interface region free of misfit dislocations. The transition from InAs to GaAs occurs gradually across 15-20 planes. The polarity of the nanowires is As-terminated and it does not change during the transition from InAs to GaAs.