Adsorption process of methylchloride (CH3Cl) on the GaAs (0 0 1)-2 x 4 surface was studied by a scanning tunnelling microscopy (STM) measurement. The arsenic rich 2 x 4 surface, which was prepared by molecular beam epitaxy (MBE), was exposed to a supersonic molecular beam of CH3Cl with a kinetic energy of 0.06 eV. New bright spots appeared on the CH3Cl exposed surface. They were largely observed at the "B-type" step edge and divided into two types according to their locations. It was suggested that new spots were due to weakly adsorbed CH3Cl molecules without any dissociation. The adsorption mechanism of CH3Cl molecule was also studied by an ab initio Hartree-Fock calculation, which explained the experimental results well. (c) 2007 Elsevier B.V. All rights reserved.