In the present study, the effect of phase behavior of sulfonic acid-functionalized phosphonium and imidazolium ionic liquid catalysts (ILCs) with reactants, ester products, and water by-product was investigated for the synthesis of various wax esters. When a relatively large amount of ILC (30 mol% to the reactant) was used for the esterification reaction, the ester product was phase-separated from the ILC, while produced water by-product was absorbed in the ILC phase. Such phase behavior effectively promoted the equilibrium reaction and gave the higher conversion (X) value as compared to the reaction systems where ILCs were homogeneously mixed with products. In the case that the concentration of ILC decreased to the catalytic amount (1 mol% to the reactant), the presence of excess water by-product altered the partition of ILC, which was strongly dependent on the structure of ILC. Although the X value became lower upon decreasing the amount of ILC used, suitable removal of water by-product via dry N-2 gas bubbling into the reaction system provided higher X values over 99%, even in the case of low ILC concentration. (C) 2019 Elsevier B.V. All rights reserved.