It is well known that overpressurized gas bubbles in solids will grow and equilibrate by acquisition of vacancies from nearby free surfaces, grain boundaries and other vacancy sources. Recently, Evans (1994,J. nucl. Mater.,210, 21) has suggested that the transfer of intragranular gas to grain boundaries in UO2may be substantially enhanced by gas bubble diffusion biased towards the grain boundaries by the flow of thermal vacancies from the boundaries into the vacancy-depleted grain interiors. To address this possibility, upper and lower bounds on the velocity of a gas bubble towards a vacancy source are determined by calculating the vacancy flux through an immobile bubble fixed with respect to the lattice, and the flux through a bubble moving with the host material respectively.