A vertical ball mill (VBM) reactor was evaluated for use in biomass conversion processes. The effects of agitation speed (100-200 rpm), number of glass spheres (0-30 units) and temperature (40-46 degrees) on enzymatic hydrolysis of rice straw and on glucose fermentation by a thermotolerant Kluyveromyces marxianus strain were separately studied. The results revealed an important role of the spheres during biomass' fiber liquefaction and yeast's fermentative performance. For hydrolysis, the spheres were the only variable with significant positive impact on cellulose conversion, while for fermentation all the variables have influenced the ethanol volumetric productivity (Q(p)). For Q(p), the spheres showed an interactive effect with temperature, being obtained a maximum of 2.16 g/L.h when both variables were used in the lowest level. By applying the needed adjustments on the levels of the variables for each process (hydrolysis and fermentation), the VBM reactor could be efficiently used for biomass conversion into ethanol. (C) 2017 Elsevier Ltd. All rights reserved.