The production of low-cost cellulase enzyme is a key step in the development of an enzymatic-based process for conversion of lignocellulosic biomass to ethanol. Although abundant information is available on cellulase production, little of this work has examined oxygen transfer. We investigated oxygen transfer during the growth of Trichoderma reesei, a cellulase-producing microorganism, on soluble and insoluble substrates in vessel sizes from 7 to 9000 L. Oxygen uptake rates and volumetric mass transfer coefficients (k(L)a) were determined using mass spectroscopy to measure off gas composition. Experimentally measured k(L)a values were found to compare favorably with a k(L)a correlation available in the literature for a non-Newtonian fermentation broth during the period of heavy cell growth.