Interdiffusion coefficients have been measured in Cu&sngbnd;Pd alloys containing 70–90 at.% Pd at temperatures from 355° to 440°C using films containing short wavelength composition modulations produced by evaporation. From the relative intensities of the high‐ and low‐angle satellites about the 111 Bragg peak, it was established that the composition modulations were coherent for &lgr;<28 Å (where &lgr; is the wavelength of the modulation) and were incoherent for &lgr;>38 Å. Over the range 28 to 38 Å, there was a progressive loss in coherency and this was accompanied by a twofold decrease in the diffusivity. This decrease confirms quantitatively the effect of coherency strains on diffusion predicted by Cahn. The dependence of the effective diffusion coefficient on wavelength in the coherent modulations yieldedK=−10.3×10−6erg·cm−1for the gradient‐energy coefficient.