Metal films of Ag, Cu, Ni, and Mo were deposited on Kapton H and spun deposited Pyralin 2540 polyimides. They were thermally treated for 0.25 to 4 h in vacuum, nitrogen (with about 1% O2), or air in the temperature range of 375 to 648 K. Their diffusion behavior was determined by Rutherford backscattering spectrometry. Ni and Mo showed excellent thermal stability, irrespective of the ambient atmosphere. Cu and Ag were, however, strongly influenced by the atmosphere during thermal treatment. Diffusion coefficient D values were calculated using the complementary error function that showed good correlation with the experimental data. The D values for Cu and Ag remained in the 10−15 cm2/s range when heated in vacuum. In the N2/O2 and air atmospheres, Ag and Cu exhibited higher diffusion at temperatures in excess of about 548 K. Ag D values showed a marked transition at 540–560 K, while Cu D values gradually increased with temperature. Cu D values increased into the 10−14 cm2/s range while that of Ag increased more rapidly to 9×10−13 cm2/s at 613 K. The behavior of all the films was independent of the polyimide (Kapton H or Pyralin 2540) or the film thickness.