The interfacial reaction between Pt films and (100)-oriented n-type GaAs substrates in the temperature range between 350 and 500°C has been studied by combining transmission electron diffraction and microscopy, glancing-incidence x-ray diffraction, and Rutherford backscattering spectroscopy. The reaction has produced PtGa and PtAs2. The phase PtAs2 has shown a strong preferred orientation on (100)GaAs and it is the phase which dominates the contact to GaAs. The orientation relation has been analyzed by using stereographic projections. Effects of the reaction on the Schottky barrier behavior have been monitored by a combination of current-voltage, capacitance-voltage, and photoresponse measurements. The value of the Schottky barrier height has been determined to be 0.9 eV and no strong variation of the barrier height with annealing has been observed.