Background and objectives: From the production of instant coffee or espresso beverages a solid waste known as spent coffee grounds (SCG) is generated. SGC are a rich source of antioxidant compounds, mainly polyphenols, which can be easily recovered by solvent extraction. However, little attention has been paid to investigating the extraction process in a systematic way. In this study, a rigorous approach based on response surface methodology is used to optimize the recovery of polyphenols from SCG and to evaluate the properties of the dry phenolic extracts obtained under optimal process conditions. Methods: Polyphenols were recovered from SCG by an extraction procedure using aqueous ethanol as the solvent. A central composite design coupled with response surface methodology was used to evaluate the effect of temperature (T = 20-60 degrees C), extraction time (E = 30-150 min), liquid-to-solid ratio (R = 10-50 mL/g) and ethanol concentration in the solvent (C = 10-90 vol%) on the extraction yield (y). A mechanically-stirred extractor operated under optimized conditions was used to obtain dry phenolic extracts. Results: All the factors examined had a positive effect on polyphenol recovery, with T and R being the most influential. The following optimal conditions were found: T = 47.1 degrees C, E = 150 min, R = 47.98 mL/g and C = 57.7%, with y(max) = 97.8%. The antioxidant activity of the dry phenolic extract, determined by the ABTS assay and expressed as Trolox equivalents (TE), was 1583 +/- 57 mu mol TE/g. Conclusions: The obtained results show that SCG from the production of espresso coffee beverages are a rich source of polyphenols. High extraction efficiencies can be achieved by optimization of the extraction process, which can be used to produce dry phenolic extracts with very high antioxidant activity. (C) 2015 Elsevier B.V. All rights reserved.