We have used normal incidence reflectivity and reflection anisotropy spectroscopy (RAS) to study the initial period of chemical beam epitaxy (CBE) growth of GaAs using either triethylgallium (TEGa) or trimethylgallium (TMGa) as group III precursors. With TEGa as the precursor, a transitory period, which can last for tens of seconds depending on the III/V ratio, is observed in both R and RAS before normal growth is established. With TMGa there is no transitory period evident in the R measurements for any III/V ratio and only a short transitory period exists in the RA measurements. These results are discussed in terms of the formation of a metallic-like surface addlayer, which subsequently becomes incorporated when using TEGa, in marked contrast to the self-limiting behaviour when using TMGa. (C)