Remarkably rapid Brownian motion of helium bubbles in aluminum alloyed with very low concentrations of lead and of indium has been observed in situ by hot-stage transmission electron microscopy. Bubble diffusion is enhanced as the impurity coats the bubble surfaces at annealing temperatures above the melting point of the impurity. The bubble diffusion coefficients calculated from video images of the bubble motion, and the corresponding surface diffusion coefficients, are orders of magnitude larger than diffusion coefficients determined from helium bubble growth experiments in pure aluminum.PACS numbers: 66.30.Jt, 66.30.Lw Helium atoms, being essentially insoluble in all metals [l], will agglomerate into bubbles that cause a deterioration of material properties. As such, the behavior of helium and other inert gases in fusion and fission reactor materials [where helium and heavier gas atoms are produced by the in,a) reaction and as fission products, respectively] is of considerable interest [2], particularly as reactor lifetimes are determined entirely by the structural damage due to gas bubbles and voids. Under such extreme conditions, bubble growth occurs as diffusing gas atoms, which produced continuously during irradiation, are captured, and by thermal-gradient-induced bubble migration and coalescence. In the absence of a continuous source of gas atoms and without temperature gradients, however, growth occurs by random bubble migration and coalescence, and, to a lesser extent, by Ostwald ripening [3].In this Letter we report in situ observations by hotstage transmission electron microscopy (TEM) of anomalously rapid helium bubble diffusion in aluminum containing a low concentration of lead, and in aluminum with a low concentration of indium, at annealing temperatures above the melting point of the impurity species. This work was motivated by recent TEM and positron lifetime spectroscopy measurements indicating surprisingly rapid helium bubble growth in aluminum containing approximately 1 ppm by weight of lead [4,5]. Figure 1, adapted from Ref.[4], shows bubble growth as a function of annealing time at 823 K for both the Pb-containing and Pb-free samples, and suggests that the accelerated bubble growth in the case of the dilute alloy may be due to extension of the migration-coalescence growth regime to much larger bubble sizes. Electron micrographs and energy dispersive x-ray spectra taken at room temperature show the lead to be present as small precipitates at preferred facets of the helium bubbles.To verify this hypothesis, samples of 99.999%-pure aluminum alloyed with 200 ppm by weight of lead, and separately, with 1000 ppm by weight of indium, were thinned to electron transparency and subsequently irradi-ated with 50-keV alpha particles at the HVEM-Tandem Facility at Argonne National Laboratory to produce a helium concentration of approximately 20 atomic ppm. The implanted samples were then annealed at 723-743 K for several minutes, during which time video recordings were made of spherical helium bubbles ...