The microstructural changes and interfacial reactions of Pt/Ti/SiNx/Si during annealing were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD), and Auger electron spectroscopy (AES). Pt/Ti thin films were deposited on SiNx/Si substrates by a d.c. magnetron sputtering system. Pt/Ti/SiNx/Si specimens were annealed at 600°C in an oxygen ambient or a vacuum. Annealing ambients strongly influenced the microstructural change in the Pt/Ti thin films. When the specimens were annealed in oxygen, the oxygen diffused into metal layers from the ambient and reacted with Ti to form the rutile phase. When the specimens were annealed in vacuum, Ti reacted with nitrogen which was dissociated from SiNx to form TiN and reacted with Pt to form Pt3Ti.