The activation volume &Dgr;Vfor chemical diffusion in Ag50Au50alloy has been determined from the pressure dependence of the rate of decrease in amplitude of composition‐modulated thin films. Based on resistivity measurements, it is argued that the technique yields the bulk chemical diffusion coefficient, in close agreement with the experimental results. Eight measurements of the diffusion coefficient were made, four as a function of temperature (237–309 °C) at atmospheric pressure and four as a function of pressure (0–0.85 GPa) at constant temperature (269.5 °C). The samples, 2000–3000‐A˚‐thick multilayers of alternating silver and gold, with periods between 72 and 160 A˚, were made with a newly built UHV sputter‐deposition apparatus. The bulk chemical interdiffusion activation energy (1.5 eV) and activation volume (0.72 &OHgr;) agree with those expected from other interdiffusion and tracer measurements.