Gas hold-up structure in aerated viscous media (mu > 0.1 Pa s), which is distinctively characterised by a nearly biomodal bubble size distribution, has been investigated in an impeller agitated reactor having a standard geometric configuration. Experiments were performed with aqueous solutions of CMC, castor oil and rapeseed oil in a glass vessel of 0.3 m internal diameter agitated by a standard six-bladed disc turbine. Large bubbles, some as large as the impeller, were formed, while tiny bubbles (d(t) = 0.1-3 mm) were found to accumulate for a while during aeration. As a result, the gas hold-up was found to vary with time. The dynamic variation of tiny bubble hold-up could be described by the equation epsilon(t) = epsilon(tf)(1 - e(-t/tau)), epsilon(tf) being the steady-state hold-up. The characteristic time constant, tau, was evaluated from this equation and its value was found to depend on the rheological and interfacial properties of liquids, impeller speed and gas velocity. Tiny bubbles were found to constitute as high as 70-80% of the total gas hold-up, and their contribution was also found to be a function of the aforementioned parameters. The effect of the formation of tiny bubbles on mass transfer rates, in particular oxygen transfer rates in an aerobic bioreactor, has been discussed. Earlier researchers who have recognised the formation of tiny bubbles in such highly viscous media have tended to assume that these bubbles have sufficiently long residence times which enable them to attain an oxygen partial pressure which is in equilibrium with the dissolved oxygen level in the liquid. It now appears that this may not be the case. Tiny bubbles do actively contribute to oxygen transfer, and their contribution is more significant in impeller-agitated reactors than in bubble columns. A theoretical framework to establish when the contribution of tiny bubbles to oxygen transfer can be significant is presented. This has been done by defining a new dimensionless group N(D) = (k(L)a)t(tau)RT/epsilon(tf)H* where (k(L)a)t is the mass transfer coefficient due to tiny bubbles, H* is the Henry’s constant at the temperature T, and R is the universal gas constant. High values of N(D) would imply that the tiny bubbles have attained equilibrium, while low values indicate that they are actively transferring solute. An expression for the ratio rate of oxygen transfer from tiny bubbles to that from large bubbles is deduced in terms of N(D).