Washing waters of Spanish-style green olive processing are heavily contaminated waste streams that represent an important environmental problem that needs to be solved. These Liquids are only generated for 2 months a year and a storage period is needed if high-value substances are to be extracted from them. Fermentation of the washing waters under acidic conditions was studied throughout a year. A lactic acid fermentation took place in vessels with either no initial correction of pH, or pH corrected to 4. When the pH was initially lowered to 3, only yeasts grew and a significant concentration of ethanol was generated. The concentration of phenolic compounds decreased slightly during the fermentation process. particularly hydroxytyrosol, which was found in high concentration in these waste waters. If this orthodiphenol is to be recovered from washing waters, time is required to hydrolyse the elenolic acid glucoside. The glucoside was hydrolysed under the acidic conditions of the washing waters, and the reaction was affected by temperature and pH. Thus, fermentation of washing waters under certain conditions may give rise to solutions with no off-odours and a high concentration of lactic acid and hydroxytyrosol, which are economically interesting products.