In this work, liquid-liquid equilibrium data for 1-ethyl-3-methylimidazolium diethylphosphate ([emiml[DEtPO4]) or 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO4]) + 4-methyl-dibenzothiophene (4-MDBT) + n-dodecane systems at 25 and 40 degrees C and atmospheric pressure (approximate to 95 kPa) were determined by refractometry. 4-MDBT is a DBT derivative and it is one of the most difficult diesel sulfur pollutants to remove by the conventional process of hydrodesulfurization. The liquid-liquid equilibrium data were used to study the 4-MDBT extraction from n-dodecane as model diesel oil. 4-MDBT distribution coefficients, solvent selectivities, and extraction data also indicate that [emim][DEtPO4] is a better solvent for extractive desulfurization of n-dodecane than [emim][EtSO4]. For a solvent/n-dodecane mass ratio of 0.6, the sulfur content in n-dodecane decreases 17-24% and 5-15% for [emim][DEtPO4] and [emim][EtSO4]. respectively. The quality of the data was ascertained by the Hand and Othmer-Tobias correlations, which presented R-2 > 0.97 for all systems. The NRTL model was used to correlate the data and showed root-mean-square deviations of <0.15%.