NH3 adsorption and dissociation on gallium-rich GaAs(0 0 1)-4 x 2 surface have been investigated using hybrid density functional theory. NH3 is found to adsorb molecularly onto GaAs(0 0 1)-4 x 2 via formation of a dative bond with an adsorption energy of 21.1 kcaV mol. Two different dissociation paths, one hydrogen atom detached from NH3 transfer to the second-layer arsenic atom and insertion into the Ga-Ga dimer bond are subsequently discussed. Optimized geometries and potential energy surfaces along both reaction paths are presented. The calculation results show both reactions are thermodynamically favorable, although not kinetically favored. (C) 2007 Published by Elsevier B.V.