The interdiffusion of platinum and gold films, a couple utilized in beam-lead microcircuits, has been studied for temperatures up to 550 °C, Gold-on-platinum couples and separate platinum and gold films, 80–450 nm thick, were deposited by electron beam evaporation onto oxidized (111) silicon substrates. Diffusion was monitored by means of spectral reflectance versus wavelength in the band 500–1000 nm. The separate metal films showed good adhesion and stable reflectances (after an initial change) for at least 6 h at diffusion temperatures, in contrast to the couples. Analysis of platinum diffusion through the gold films yielded an activation energy about 38 kcal/g-atom and a pre-exponential factor of the order 10−3 cm2/sec, values close to those for volume diffusion. The pre-exponential factor especially is dependent upon film deposition conditions.