The traditional production of long-chain organic esters like isobutyl isobutyrate (BIBE) suffers from severe problems due to the homogenous catalyst in process and complex system thermodynamics. In this work, an innovative bifunctional deep eutectic solvent (DES) is introduced, playing as both reaction catalyst and extraction solvent, to intensify the esterification process. The DES was formed by imidazole (Im) and p-toluenesulfonic acid (PTSA), which was for the first time found acid-base tunable, represented by weak basic DES [3Im:PTSA] and strong acidic DES [Im:2PTSA]. The formation mechanisms of [3Im:PTSA] and [Im:2PTSA] were illustrated by Fourier-transform infrared spectroscopy (FTIR) and COnductor-like Screening MOdel for Real Solvent (COSMO-RS) combined with the Im-PTSA phase diagram. The dual functions of the acidic [Im:2PTSA] in reactive extraction were evaluated by sigma-potential analysis and esterification experiments. The high conversion, easy product separation, and good solvent reusability confirm the excellent catalytic and solvent effect of [Im:2PTSA]. (c) 2018 American Institute of Chemical Engineers AIChE J, 65: 675-683, 2019