Demarcation of large and small bubbles was investigated in viscous slurry bubble columns of which diameter was either 0.051, 0.076, 0.102, or 0.152 and 1.5 m in height. The bubble size demarcating the large and small bubbles was considered with variations of gas velocity, liquid viscosity, solid content in the slurry phase, and column diameter. The size (chord length), rising velocity, and holdup of bubbles were measured by means of the dual-electrical resistivity probe (DRP) method, and the holdups of large and small bubbles were determined by adopting the dynamic gas disengagement (DGD) method. The holdups of large and small bubbles which were determined by means of two different methods were compared to each other to find out the bubble size for the demarcation between the large and small bubbles. In addition, the bubble rising velocities were compared with the interstitial gas velocities based on the holdups of bubbles larger than the indicated bubble size, to look for the demarcating bubble size between the large and the small bubbles. It was found that the bubbles of which size were larger than 5.0-6.0 x 10(-3) m can be termed as large bubbles while others as small bubbles, respectively. The holdups of large and small bubbles are correlated in terms of operating variables, respectively, within these experimental conditions.