[en] Rate-controlling mechanisms for bubble migration in temperatures below 0.6Tsub(m) have been studied. An extension of Gruber's model (1967) takes account of the constant gas pressure in the bubbles (likely at low temperatures with no continuous radiation damage) and the possibility that the nucleation of a surface ledge can control the migration rate of faceted bubbles. The experimental growth rates of helium bubbles, measured in niobium, niobium-zirconium alloys and vanadium, are shown to be consistent with bubble migration by a surface diffusion mechanism controlled by the surface diffusion coefficient for small bubbles but by ledge nucleation for larger bubbles. The bubble size above which the (slow) ledge nucleation process controls growth is sensitively affected by the ledge energy. The addition of zirconium to niobium can alter the ledge energy by an order of magnitude by cleansing the bubble faces of oxygen. Subsequent segregation of Zr-O complexes to the bubbles further alters the ledge energy. The bubble growth rate, and hence the swelling and embrittlement behaviour of the material under these conditions, is very sensitive to the material purity and to segregation effects either induced thermally or accelerated by transmutation and irradiation damage. (U.K.)