A system and method for mixing high viscous liquids with gas is provided. The disclosed embodiments include a reactor or mixing vessel having a draft tube disposed therein, a gas injection subsystem adapted to inject gas into the reactor or mixing vessel proximate the entrance of the draft tube. The embodiments also include an agitator disposed within the draft tube which makes the draft tube the primary site for the gas-liquid mixing. In particular, the agitator is adapted to create gas bubbles having an average diameter between about 0.3 mm and 3.0 mm which are then ejected into the reactor or mixing vessel. The mass transfer efficiency associated with the present system and method is enhanced from the combined effect of gas dissolution into the high viscous liquid within the draft tube and greater bubble residence time within the high viscous liquid.