The resistivity and residual resistance ratio of 31-nm films of AuIn 2+δ have been measured for a series of precision vacuum-deposited films covering the range of -0.04≤δ≤0.12. Both the resistivity and the resistance ratio exhibit a strongly peaked dependence on δ around δ=0. The results are analyzed in terms of the Mayadas-Shatzkes theory of scattering in thin films which includes the effects of both surface and grain boundary scattering. The dominant contribution to the resistivity is due to grain boundary scattering. The grain boundary scattering coefficient is unusually large compared to published results on single-phase metal films. These findings are interpreted in relation to the structure of the films and to the presence of second-phase material (AuIn or In) in the grain boundaries.