A comparative study of GaAs (001) growth from tri-methyl gallium (TMGa) and tri-ethyl gallium (TEGa) was performed in metal-organic vapour phase epitaxy. The growth surfaces were characterised by reflectance anisotropy spectroscopy (RAS). Three distinct shapes of the RAS spectra typical for certain growth parameter regions were observed for both precursors. The RAS spectra typical for the growth at low temperatures correspond to kinetic limited growth and the surface is covered by adsorbates. At slightly higher temperatures in mass transport limited growth mode, a more gallium-rich surface appears with both precursors. In the case of TEGa, the lack of arsenic on the surface below 500degreesC is simply due to the lack of decomposed AsH3. However, using TMGa gallium-rich surfaces are already found below 600degreesC. We propose steric hindrance of As diffusion by methyl groups around the incorporation sites and forced arsenic desorption by the formation of methyl arsine as the cause. At typical buffer growth temperatures above 600degreesC, the surfaces during growth are identical to the AsH3 stabilised arsenic-rich pre-growth surfaces for both gallium precursors. (C) 2003 Elsevier B.V. All rights reserved.