The steady state mass transfer coefficient, k(L)a, and the hold-up, epsilon, have been measured in a vessel 0.29 m in diameter, sparged with air using 6RT (single and dual) or 6SRGT (single) impellers at unaerated energy dissipation rates of up to similar to100 kW m(-3) and superficial gas velocities of up to 0.13 ms(-1) in deionized water (coalescing) and an aqueous solution of 0.2 M-1 sodium sulphate (non-coalescing). Under these intense conditions, the flow structure changed from homogeneous to heterogeneous, the hold-up could be as high as similar to65% and the gassed to ungassed power ratio for the RT and 6SRGT as low as 0.2 and 0.6, respectively. k(L)a values up to similar to2.0 s(-1) were found and the results could be correlated well by k(L)a = k(P/V-L)V-a(S)beta vs with a single parameter set for each fluid, based on a 'well-mixed' model and a chi(2)-method of data fitting.