The precipitation of arsenic in GaAs epitaxially grown at low temperature (LT GaAs) has been studied as a function of the post-growth annealing temperature by three independent methods: transmission electron microscopy (TEM), Raman scattering, and for the first time the anomalous small angle X-ray scattering (ASAXS). All the results agree that the average size increases with increasing annealing temperature but the ASAXS data indicate that the precipitates can be divided into two parts described by separate size distributions. The number density of precipitates around 5 nm size has been estimated to be at least two orders of magnitude higher than that of larger precipitates.