Isobaric vapor-liquid equilibrium at 101.3 kPa for the ternary system methanol + dimethyl carbonate +1-butyl-3-methylimidazolium dibutylphosphate ([BMIM][DBP]) and their binary systems are determined using a modified Othmer still. By adding [BMIM][DBP] into the azeotropic system of methanol + dimethyl carbonate, the relative volatility of dimethyl carbonate is increased, which might be ascribed to the salting-out effect of [BMIM][DBP]. The relative volatility alpha(21) increases with increasing molar fraction of [BMIM][DBP]. The azeotropic point disappears when the molar fraction of [BMIM][DBP] is above 0.150. The equilibrium data are well fitted by the electrolyte nonrandom two-liquid model.