Unintentional As/P exchange has a significant influence on InAs/InP nanostructures growth. In this paper, we report on the As/P exchange reactions at different temperatures studied by in situ stress measurements and reflectance difference (RD) characterization. When arsenic atoms are incorporated at the InP surface, an asymmetric stress is built up that is responsible for quantum wires (QWr) formation. We obtain that arsenic atoms do not actively displace phosphorous from the surface at the range of surface temperatures and exposure times of interest for growth of InAs nanostructures by molecular beam epitaxy (MBE). Instead, the arsenic atoms bind to available In atoms left vacant by phosphorous desorption at the corresponding temperature. (C) 2002 Elsevier Science B.V. All rights reserved.