This paper postulates four possible mechanisms for the enhancement of gas-liq. (G-L) mass transfer in slurry reactors: (1) hydrodynamic effect, (2) shuttling, (3) stabilization of bubbles, and (4) reaction enhancement. Mass transfer and reactivity expts. using two different slurry systems and two different Pd-catalyzed reactions, i.e., oxidn. of glucose (aq. phase) and hydrogenation of a-Me styrene (org. phase), with both hydrophobic carbon and hydrophilic silica catalyst supports, were performed in a lab. scale surface aeration reactor with a flat gas-liq. interface. Phys. and reaction enhancement are distinguished successfully. The exptl. results show that the level of catalyst support hydrophobicity has a strong effect on the rate of mass transfer.