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.