Bubble diameter measurements in a two-dimensional cocurrent bubble column are obtained using a gas-liquid-solid system in which the solid component is a cellulose fiber. Flash X-ray radiography, a noninvasive measurement technique, is used to record bubble size in the opaque slurry at various operating conditions. Results are presented for a range of fiber mass fractions (0 less than or equal to C less than or equal to 1.5%), a range of superficial gas velocities (1 less than or equal to nu(g) less than or equal to 4 cm/s), two superficial liquid velocities (nu(1) = 1 or 2 cm/s), and two column heights (H = 15-40 or 115-140 cm). Bubbles are categorized as either large (d(B) > 10 mm) or small (d(B) less than or equal to 10 mm), and all bubble diameter distributions can be characterized by log-normal distributions. The presence of fibers has the most significant effect on the large bubble size and population, even at mass fractions as low as 0.5%. In general, the large bubble size and population increases with column height, superficial gas velocity, and fiber mass fraction.