This work reports how warm water washing of biomass affects fast pyrolysis. Specifically, we applied this concept to Arundo donax, a common herbaceous biomass, and studied the washing effects on fast pyrolysis yields in a Py-GC/MS and a fluidized bed reactor. We stir-washed samples in water at 20 degrees C, 40 degrees C and 60 degrees C, and found that water at 60 degrees C led to an 18% increase in the removal of ash and an 11% increase in the removal of potassium compared to a room temperature wash. We pyrolyzed these samples via Py-GC/MS-FID and found that the reduction in ash increased the pyrolysis yields of acetol, furanone and levoglucosan relative to the unwashed samples. Additionally, we pyrolyzed washed A. donax in a fluidized bed reactor and compared the results to those of unwashed A. donax. We found that A. donax washed at 60 degrees C produced 9.7 wt% more bio-oil than unwashed, with increases in glycolaldehyde and levoglucosan yields of 0.78 wt% and 1.24 wt%, respectively. The yield of olefinic gases increased by 0.22 wt% and yields of carbon dioxide and alkane gases decreased 2.40 wt% and 0.11 wt% respectively. Char and water yields decreased 4.32 wt% and 10.08 wt % respectively. The gas produced from pyrolysis of washed A. donax has a greater HHV than that from unwashed A. donax (7.77 MJ/kg vs. 6.81 MJ/kg). We believe these results are a consequence of differences in the extent of secondary reactions catalyzed by the inorganic species present in herbaceous biomass.