Single bubbles of very pure N-2, He, air and CO2 were formed in a quiescent environment in ultra-clean water, with diameters ranging from 10 to 100 mu m. Their terminal rise velocities were measured by high-speed video microscopy. For N-2, He and air, excellent agreement with the Hadamard-Rybczynski (H-R) equation was observed, indicating that slip was occurring at the liquid-vapor interface. For CO2 bubbles with diameters less than 60 mu m, the terminal rise velocities exceeded those predicted by the H-R equation. This effect was ascribed to the enhanced solubility of CO2 compared with the other gases examined. The presence of a diffusion boundary layer may be responsible for the increased terminal velocity of very small CO2 bubbles. Crown Copyright (C) 2008 Published by Elsevier Inc. All rights reserved.