Results in a previous study showed up to a 55% increase in saccharification rates when the initial particle size range decreased from 590 < x < 850 microm down to 33 < x < 75 microm. The smaller particle sizes also lowered the viscosity of the slurry 50-fold (for an equivalent initial solids concentration). In this study, ultrasonic irradiation was employed to further reduce the particle size of sawdust slurries below the ranges in the previous study in an attempt to further increase enzymatic saccharification rates and lower the slurry viscosity. Average particle sizes were reduced to less than 1 microm under the conditions tested. Surprisingly, the amount and rates of sugar released in this study with the approximately 1 microm particles is comparable (maximum glucose release of 30%) to, but no better than that seen for particle sizes in the range of 33 < or = x < or = 75 microm (maximum glucose release of 31%). Also surprisingly, the viscosity increased as the average particle sizes in the slurries decreased, which is opposite to the trend in the previous study. For example, there was an approximately threefold increase in the viscosity between unsonicated samples with a range of 10 < or = x < or = 75 microm and sonicated samples with a range of 0.05 < or = x < or = 12 microm. This is attributed to the variations in surface characteristics of the particles which were characterized here using X-ray diffraction profiles and SEM pictures.