GaAs nanowires were grown on GaAs (111)B substrates in a gas source molecular beam epitaxy system, using self-assembled An particles with diameters between 20 and 800 nm as catalytic agents. The growth kinetics of the wires was investigated for substrate temperatures between 500 and 600 degrees C, and V/III flux ratios of 1.5 and 2.3. The broad distribution of Au particles enabled the first observation of two distinct growth regimes related to the size of the catalyst. The origins of this transition are discussed in terms of the various mass transport mechanisms that drive the wire growth. Diffusion of the growth species on the 2-D surface and up the wire sidewalls dominates for catalyst diameters smaller than similar to 130 nm on average, while direct impingement on the catalyst followed by bulk diffusion through the Au particle appears to sustain the wire growth for larger catalyst diameters. A change in wire sidewall facets, indicating a probable transition in the crystal structure, is found to be primarily dependent on the V/III flux ratio. (c) 2005 Elsevier B.V. All rights reserved.