BACKGROUND: In recent years enantioselective esterification of racemic ibuprofen performed in organic co-solvent media such as isooctane and cyclohexane and catalyzed by lipases, has been proposed as an effective way to increase the concentration of S-ibuprofen in the racemic mixture. In this contribution, the enantioselective enzymatic esterification of (R,S)-ibuprofen with ethanol catalyzed by commercial Novozym 435 without the addition of a co-solvent is thoroughly investigated. Experimental data are further analyzed considering the results of extensive molecular modeling calculations.