The gas-liquid interfacial area and mass transfer coefficient for absorption of oxygen from air into water, aqueous glycerol solutions up to 1.5% (w/w) and fermentation medium containing glucose up to a 3% concentration were determined in a co-current down flow contacting column (CDCC; 0.05 m i.d. and 0.8 m length). Experimental studies were conducted using various nozzle diameters at different gas and re-circulation liquid rates. Specific interfacial area (a) is determined from the fractional gas hold-up (80 and the average bubble diameter (db). Once the interfacial area is determined, the volumetric mass transfer coefficient (k(L)a) is then used to evaluate the film mass transfer coefficient in the CDCC. The effects of operating conditions and liquid properties on the specific interfacial area were investigated. The values of interfacial area in air-aqueous glycerol solutions and fermentation media were found to be lower than those in the air-water system. As far as experimental conditions were concerned, the values of interfacial area obtained from this study were found to be considerably higher than those of the literature values of conventional bubble columns. The penetration theory is used to interpret the film mass transfer coefficient and results match the experimental k(L) data reasonably wen. (c) 2006 Society of Chemical Industry