The parameters, including ultrasonic frequency, still versus stirring, biomass particle size and biomass loading were concurrently investigated for the ultrasonic treatment of wheat straw. Experiments were conducted at three different frequencies; 40, 376, and 995 kHz using three different solid to liquid ratios, 1/50, 1/20, and 1/15 (g/ml), with and without mechanical stirring. Additional treatments in different particle size ranges, 0-0.5, 0.5-1, and 1-2 mm were performed at the solid to liquid ratio of 1/20 (g/ml). Fractionation was improved at 40 and 995 kHz via different mechanisms. Delignification was favored at the ultrasonic treatment frequency of 40 kHz, biomass loading 1/20 (g/ml) with stirring and particle size range of 0.5-1 mm. However at 995 kHz carbohydrate solubilization was favored, especially in the particle size range of <0.5 mm. The treatment efficacies highlighted the use of ultrasound for physical and chemical effects at different frequencies. (C) 2013 Elsevier Ltd. All rights reserved.