Reflectance anisotropy spectroscopy (RAS) has been used to investigate the As/P exchange reaction for group V stabilized InP(001) surfaces exposed toAs2and/orP2, under molecular beam epitaxy conditions. By comparing RAS spectra taken before, during, and afterAs2exposure it is possible to confirm that the As/P exchange reaction is exactly reversible over a range of temperatures from 420 to 560 °C. Time-resolved RAS measurements of the reaction rate, monitored at an energy of 2.65 eV, indicate that the activation energy for the exchange is 1.23±0.05 eV. ©1997 American Institute of Physics.