We have investigated the amorphous phase formation and initial crystalline reactions at Pt/GaAs interfaces via high‐resolution transmission electron microscopy (HRTEM) andinsituHRTEM. A 3‐nm‐thick amorphous intermixed layer consisting of three elements, platinum, gallium, and arsenic formed at the Pt/GaAs interface during the deposition of a 500‐A˚‐thick Pt film. The interlayer grew in a planar fashion in an amorphous state upon low temperature (e.g., 200 °C) annealing by a solid‐state amorphization reaction. This reaction occurs with a driving force of a negative heat of mixing, and by the dominant diffusion of Pt to the GaAs substrate, which was verified byinsituHRTEM. Following the growth of the amorphous interlayer, the Pt3Ga and PtAs2phases nucleated within the amorphous layer and grew at the Pt and GaAs sides, respectively. The relative mobility of the three constituents at the low temperature, the structure of the crystalline intermetallic compounds, and local thermodynamical equilibrium are responsible for the sequence of the crystalline phase formation. After a complete reaction at 400 °C for 20 min, we observed the formation of a layered structure of PtGa/PtAs2/GaAs as the final structure.InsituHRTEM experiments also demonstrated growth of the amorphous intermixed layer and crystalline reaction between the Pt film and the GaAs, which is consistent with the results from theexsituannealing treatment.