Lateral diffusion of In in 20–100‐nm‐thick Au‐In films has been studied in the 60–150 °C temperature range. Indium diffusion due to the chemical potential gradient occurs from In to Au‐In with an AuIn2phase being formed in the diffusion zone. The growth of the diffusion zone obeys a parabolic rate law with the growth rate strongly dependent on In concentration of the films. In order to understand these phenomena, the kinetics of the AuIn2phase formation is analyzed considering In diffusion due to the chemical potential gradient with subsequent formation of AuIn2at the diffusion front. As a result of this analysis, the diffusion coefficient of In in AuIn2films is obtained asDIn=1.2×10−3 exp(−0.52 eV/kT) cm2/s. Grain boundary diffusion is thought to be the dominant mechanism. Additionally, a combined diffusion parameter &dgr;Dg(&dgr; is the grain boundary width) of 1.3×10−16cm3/s (60 °C) is obtained.