The hydrothermal pretreatment of olive mill solid waste amended with 0.6 M organic acids was studied at temperatures between 100 and 170 degrees C. Acetic and formic acids which are endogenous intermediates of hemiacetyl splitting at subcritical conditions were tested. Formic acid, with smaller molecular size and lower pKa, was found to be more effective than acetic in the entire range of temperatures tested. Yield of enzymatic hydrolysis was significantly enhanced (>2 folds) at temperatures above 140 degrees C. Concentration of aldehyde byproducts in the medium increased with temperature and pressure and addition of organic acids, however, the highest concentration detected (ca 1 g/L) did not surpass values reported as inhibitory of sugars fermentation to ethanol by either yeast or bacteria. Aldehyde production was more affected by temperature than by acid addition. Concluding, addition of formic acid to hydrothermal pretreatment at relatively mild temperatures (140-170 degrees C) and pressure (10-13 atm) improved saccharification yield while saving energy. (C) 2015 Elsevier Ltd. All rights reserved.