Copper-nickel thin films with composition modulation wavelengths of 9 Å and 35 Å have been prepared on thin reed substrates of fused silica. The films have been annealed at temperatures up to 550 °C to produce interdiffusion. The virtual absence of any change in the low temperature natural vibration frequencies shows that the Young's modulus of the film is essentially unaffected by homogenization, and thus no anomaly is apparent in the elastic behavior of the initial modulated structure. Above 300 °C a strong thermally activated internal friction is observed, which decreases irreversibly on annealing near 500 °C in a manner that correlates with homogenization of the film by lattice diffusion. The internal friction involves a broad spectrum of relaxation times with activation energies between 2 and 3 eV. It is concluded that grain boundary relaxation is probably the dominant cause of the internal friction, but this is only possible if the relaxation time is proportional to the grain size, as predicted by the theory of viscous sliding. Because of their much finer grain size, it is expected that grain boundary peaks may be exhibited in thin films at appreciably lower temperatures than in bulk samples. © 1976.