The application of a novel industrial process based on the hydrothermal treatment of olive oil waste (alperujo) led to a final liquid phase that contained a high concentration of simple phenolic compounds. In this study the effect of pH on phenol extraction with ethyl acetate from the aqueous phase of hydrothermally treated alperujo at 160°C/60min (without modification, pH 4.5, and adjusted to pH 2.5) was evaluated, beside the increase of hydroxytyrosol during the storage. The variation of the concentration of phenolic compounds in each extract was analyzed by HPLC. The phenolic extract obtained at pH 4.5 presented a higher proportion of total and individual phenols and better antioxidant capacity in vitro than the extract obtained at pH 2.5. The use of lower pH values enhances the concentration of hydroxytyrosol in the liquid diminishing the storage times.