The role of endocellulases and endoxylanase during liquefaction and saccharification of hydrothermally pretreated wheat straw was studied. The use of a flow-loop setup with in-line magnetic resonance imaging enabled frequent measurements of viscosity at 55 degrees C during saccharification at 6% total solids content. Viscosity data were complemented with off-line measurements of fiber lengths and release of soluble sugars. A clear correlation between fiber attrition and a decrease in viscosity was found. Fiber lengths and viscosity dropped quickly within the first hour and then stagnated, while sugar yields increased substantially thereafter, illustrating that liquefaction and saccharification are separate mechanisms. Both endoglucanase and endoxylanase were shown to have a significant effect on viscosity during liquefaction while the addition of endoxylanase also increased sugar yield. (C) 2014 American Institute of Chemical Engineers