Desulfurization of fuel is essential to be done from the environmental and industrial points of view. Extractive desulfurization (EDS) is one of the most promising methods in order to meet statutory sulfur content. Among of various extractants, ionic liquids (ILs) are more suitable ones due to their desirable and compatible properties having green chemistry disciplines. This work shows that tributyl(carboxymethyl) phosphonium bromide [TBCMP][Br], as a task-specific phosphonium ionic liquid (PIL) including acetic acid moiety, is able to extract benzothiophenic compounds from liquid fuel, successfully. A series of experiments were done to determine the effect of PIL/fuel volume ratio, time, temperature, multiple extraction and initial concentration on EDS performance of [TBCMP][Br]. The efficiency of extraction for sulfur compounds was determined and obeyed the order of dibenzothiophene > benzothiophene > dimethyldibenzothiophene at 69%, 53% and 40%, respectively. For the volume ratio 0.5 in 15 min at room temperature, deep desulfurization can obtain after four extraction times. Finally, the extraction mechanism of PIL, especially the role of acetic acid moiety, was studied experimentally and theoretically for the first time. The computational results show that the alpha hydrogens of methylene groups, connected to phosphonium core and hydrogen of acetic acid moiety, provide hydrogen bonding interaction which indicate [TBCMP][Br] acts as a bi-functional IL for EDS. So, the present work is supposed to create a new and promising insight into the courses of rational designs, syntheses and applications of task-specific ionic liquids for green desulfurization. (C) 2016 Elsevier B.V. All rights reserved.