The focused beam reflectance measurement (FBRM) probe was directly inserted into a small fermentor with a working volume of 650 mL and the on-line monitoring technique for the size distribution of SPSC01, a self-flocculating yeast strain, was established. It was found that the floc size distribution could be characterized by its mode, M, which was further correlated by a power function with the stirring speed, r. Therefore, the designated floc size distributions were obtained through controlling the stirring speed. The batch fermentation times were significantly extended when the same levels of ethanol and biomass concentrations were reached, but the floc size distributions characterized by M increased from 100 to 600 mu m. These experimental results indicated that the intra-particle mass transfer limitation existed for such a self-flocculating yeast ethanol fermentation system and the kinetics ought to be categorized into two different levels: intrinsic and observed. The impacts of the floc size distributions on the observed rates for glucose uptake and ethanol production were quantitatively investigated by manipulating the stirring speed so that the designated floc size distributions were achieved. The effectiveness factors, eta(S) and eta(P) for glucose uptake and ethanol production, respectively, were further evaluated. It was found that both the floc size distribution and the level of limiting substrate concentration affected eta(S) and eta(P). (c) 2005 Elsevier Inc. All rights reserved.