Ga diffusion in single crystal Cu(In1−xGax)Se2 (CIGS) epitaxial layers on GaAs was measured as a function of the Cu/In ratio in the CIGS. No Ga was supplied intentionally during deposition but Ga was released by the substrates into the growing film. The concentration profile of the Ga was determined by secondary ion mass spectrometry and was fit by numerical solution of Fick’s second law including motion of the surface and the possibility of Ga desorption from the surface. All films studied had Cu/(In+Ga) ratios greater than 1.0, which was determined by the Ga outdiffusion from the GaAs. The Cu/In ratio was controlled by the deposition conditions. The Ga diffusivity was a minimum near a Cu/In ratio of 1.0 and increased rapidly for both higher and lower values of this ratio. The diffusivity ranged from a minimum of 2.7×10−13 cm2/s at Cu/In=0.94 to 5×10−11 cm2/s−12 cm2/s at Cu/In=1.41 and 7×10−12 cm2/s at Cu/In=0.43. It was concluded that Ga diffuses on metal vacancies on either the Cu or In sublattice.