The interdiffusion and intermetallic formation in thin‐film sandwiches of platinum on preannealed aluminum has been investigated at temperatures in the range 200–500 °C. X‐ray diffractometer studies, sheet‐resistivity measurements, and microscopic studies were employed to follow the metallurgical reaction. It has been found that Al‐Pt interact very rapidly, leading to the formation of several intermetallics. In heat‐treated samples with thick aluminum (?6000 Å), Pt2Al3, PtAl2, PtAl3, and PtAl4 were present. In others with 4000 Å or less Al, platinum‐rich phases Pt5Al3 and Pt3Al2 were detected. After prolonged or high‐temperature anneals, phases comparatively richer in Al were found to grow at the expense of other phases formed earlier. The Al‐Pt interaction rate was found to be dependent on the annealing ambient, being significantly higher in forming gas, argon, or helium than in vacuum or air. Also, the intermetallic formation resulted in large stress changes in the composite Al‐Pt films due to a volume increase accompanying the formation of Al‐rich phases, which led to the loss of adhesion.