Solution potential measurements were used to determine the copper concentration at the surface of Alclad 2014 alloy sheet after thermal treatments in the 400 to 500 degrees C temperature range. Interdiffusion coefficients calculated from the concentration-time dependence were found to be 5 to 10 times greater than those for lattice diffusion; the apparent activation energy was also significantly lower than that for lattice diffusion (similar to 85 vs. 125 kJ/mol). The discrepancy was ascribed to grain boundary effects, whereby the calculated effective diffusion coefficients agreed closely with the experimental values.