In situ reflectance anisotropy spectroscopy was employed to interpret the surface reaction rate constant (k(s)) of a Ga precursor on a GaAs(001) surface in metallorganic vapor-phase epitaxy (MOVPE), which was extracted by the analysis of selective-area growth. The activation energy of ks significantly decreased above 625-630 degrees C. Correspondingly, the surface anisotropy spectrum changed around 600 degrees C from that of surface reconstruction containing Ga dimers at lower temperatures to that of c(4 x 4)-like reconstruction with As dimers at higher temperatures. These observations suggest a step-flow growth mode at higher temperatures and an island growth mode at lower temperatures. (c) 2007 The Electrochemical Society.