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The first direct measurement of the grain‐boundary ion drift velocity in thin Au films over the temperature range 120–250 °C is reported. Central regions on the narrow stripe conductors were selectively embedded with 195Au tracer atoms and the extent of the subsequent transport was evaluated by a high‐resolution autoradiography technique employing the scanning electron microscope. In one operating mode, the secondary electron image of the exposed Ag grains in the photographic emulsion was revealed. The second mode of operation involved detection of fluorescent Ag‐Lα x rays, generated upon impingement of the finely focused electron beam on the Ag grains. The resultant spatial dependence of the x‐ray intensity was ultimately converted into tracer activity profiles. It was found (1) that Ion transport in grain boundaries of Au is anode directed (this is the case whether or not a Mo glue layer is present), (2) that the ion drift velocity in grain boundaries can be expressed as V=[(5.7±0.6) ×10-2j/T] exp [-(0.80±0.03 eV)/kT] (cm/sec), with j given in units of A/cm2, (3) that the effective valence of ions migrating in grain boundaries ranged from -89 at 120 °C to -20 at 250 °C (for a diffusion correlation factor of unity), and (4) that a plateau was observed in the low temperature (≪180 °C) profiles. (At higher temperatures, the plateau was less apparent and the integrated intensity was higher). This last result is in semiquantitative agreement with the theory developed by the authors to account for grain‐boundary diffusion and electromigration, in the presence of simultaneous atom leakage into the lattice.