Characteristics of oxygen transfer were investigated in the riser of a viscous three-phase circulating fluidized bed, with a diameter of 0.102 m (1D) and a height of 3.5 m. Effects of gas (0.01-0.09 m/s) and liquid (0.12-0.43 m/s) velocities, fluidized solid particle size (1.0, 1.7, 2.1, 3.0 mm), solid circulation rate (2-8 kg/(m(2).s)) and liquid viscosity (0.96-38 mPa-s) on the volumetric oxygen transfer coefficient in the riser were examined. The oxygen transfer coefficient was found to increase with increasing gas velocity, particle size and solid circulation rate, while it decreased with increasing viscosity of the continuous liquid medium, and changed very little with the variation of liquid velocity. The oxygen transfer coefficients were well correlated in terms of dimensionless groups as well as operation variables.