The surface migration length of As-4 molecules on (4 1 1)A surfaces during molecular beam epitaxy has been determined from lateral profiles of As content (x) in GaAsxP1-x layers grown on (4 1 1)A GaAs substrates with a (1 0 0) side-slope using As-4 and P-2 molecular beams. The migration length of As-4 molecules on (4 1 1)A surfaces at a substrate temperature of 535degreesC was found to steeply increase from 4 to 10 mum and saturate to 10 mum with increasing V/III flux ratio. This V/III ratio dependence of migration length of As-4 is understood by a proposed model where an As-4 molecule is dissociated and incorporated into the crystal phase when an As-4 molecule encounters a Ga adatom on the (4 1 1)A surface. Quantitative analysis is made using rate equations based on the model and the experimental results are reproduced with reasonable fitting parameter. (C) 2002 Elsevier Science B.V. All rights reserved.