An experimental investigation of the effect of fiber type on gas holdup in a cocurrent air-water-fiber bubble column is presented. Three types of cellulose fibers (i.e., hardwood, softwood, and bleached chemithermomechanical pulp (BCTMP)) and three different lengths of Rayon fibers are used in the investigation. The results indicate fiber type has a significant influence on the gas holdup in the air-water-fiber bubble column. Mechanisms by which fibers influence gas holdup in gas-liquid-fiber bubble columns are summarized and used to explain the experimental results. Fiber physical properties, including fiber length, coarseness, and flexibility are proposed to be the main factors responsible for the fiber type effects on gas holdup. (c) 2005 Elsevier B.V. All rights reserved.