Reflectance anisotropy spectroscopy (RAS) has been used to investigate the As/P exchange reaction for group V stabilized InP(001) surfaces exposed to As-2 and/or P-2, under molecular beam epitaxy conditions. By comparing RAS spectra taken before, during, and after As, exposure it is possible to confirm that the As/P exchange reaction is exactly reversible over a range of temperatures from 420 to 560 degrees 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. (C) 1997 American Institute of Physics.