Recently, many efforts to enhance separation performance of the reverse osmosis (RO) membranes have been made. Among them, the post treatment with organic solvent, so called solvent activation, has been recognized as an effective method to improve membrane performance. However, solvent activation enhances water flux along with the loss of NaCl rejection. Furthermore, there have been no clear mechanisms and reliable criteria of the solvent activation effects. In this study, we demonstrate that a new type of organic solvent, benzyl alcohol, can effectively activate the RO membrane to significantly enhance water permeation without deteriorating NaCl. Based on this results, we elucidate the underlying solvent activation mechanism and propose a reliable indicator of the solvent activation effect.