Nowadays, the most common use of olive stones, after being removed from olive mill solid waste, is for combustion to generate heat or electric energy. So, this lignocellulosic by-product is underused when it is burnt, because of its high content in phenolic compounds. Those might be regarded as high-value-added products not only in the food industry, but also in the chemical and construction industries because they could be chemical building-blocks in the synthesis of compounds with interesting physicochemical properties (i.e., resin). The type and the concentration of these phenolic compounds vary depending on the extraction technique, since they can easily degrade and pass from a compound to another when using high temperatures during extraction.

A Green Chemistry approach was followed in this work. It means that an optimization was made by designing the experiments of the extraction process based on response surface methodology. This extraction was carried out using an unconventional method, namely, microwave-assisted extraction. As extraction solvent, aqueous acetone was used since it is environmentally safe and easy to recover. The principal phenolic compounds identified in the extracts were protocatechuic acid and hydroxytyrosol, and their optimal extraction conditions were 45 ºC, 20 min and 90% (v/v) acetone for the former, and 74.7 ºC, 19.37 min and 90% (v/v) acetone for the latter.