In this paper the solvent extraction of sugar with an ion-pair of boronate anion and quaternary ammonium cation was carried out. Extraction reactions were formulated, and extraction equilibrium constants were obtained based on experimental results. Extraction equilibrium constants, K-f, of extraction of glucose and xylose were higher than those of arabinose, fructose, galactose, mannose, and fucose. We measured the stability constant K-s, between phenylboronic acid and sugars in the aqueous solution to clarify the dominant factor involved in the extraction reaction. The order of K-s, values did not correspond to K-f values. This suggests that the lesser ability of 1,3-trans-diol for ion-pair formation with trioctylmethylammoniun and the instability of the ion-pair complex in the organic phase due to steric hindrance contribute mainly to the extraction reaction. (C) 2004 Elsevier B.V. All rights reserved.