Gas holdups, dispersion height, volumetric mass transfer coefficients and k(L)a of water and a yeast fermentation broth were studied in a co-current downflow contacting reactor. k(L)a and gas holdup increased with increasing superficial gas velocity and there was a parallelism between k(L)a and gas holdup. The values of k(L)a gas holdup, and dispersion height measured in the air/fermentation broth system were all lower than those in air/water system.