Oscillations in both the intensity and width of reflection high energy electron diffraction (RHEED) are observed to depend upon most MBE growth parameters. Previously, we reported that oscillations in the width of the specular RHEED beam could be used to measure the mean island size occuring during growth. From the mean island size, the diffusion length of adatoms on the surface was estimated. On a GaAs(100) surface the measurements are extended to determine the dependence of the surface diffusion of Ga on Ga arrival rate, As4 flux, crystal misorientation, and substrate temperature. At temperatures where Ga evaporation is not significant, but still high enough to be in the 1×1 surface phase region, we find the diffusion length decreases as the square root of the Ga arrival rate. For fixed Ga arrival rate, the correlation length was measured as a function of temperature and as the As4 flux was varied. These results are compared to those obtained by Neave et al. [Appl. Phys. Letters 47 (1985) 100].