Absorption of oxygen from air into distilled water in a new type of airlift tube reactor has been investigated at the 1.3 1 scale. Results of this mass transfer study together with hydrodynamic results provide a means of optimizing the geometrical parameters of the reactor. The overall volumetric oxygen transfer coefficient (K(L)a) was measured as a function of the gas velocity in reaction tubes, the liquid hold-up in the reactor expressed in terms of liquid height, the cross-sectional area ratio of the recycle-to-reaction tubes (A(rec)/A), the liquid temperature and specific power input. The K(L)a was measured using a dynamic gassing method with polarographic measurements of dissolved oxygen.