The equilibrium vapor pressures, p(e), of the ionic liquid tetraoctylphosphonium bis(2-ethylhexyl) phosphate ([P-8888][DEHP]) over the temperature range 409-495 K were determined for the first time using mass loss Knudsen effusion. The p(e) versus temperature relationship compares well to 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl) imide ([C(8)mim][NTf2]) but is lower than that of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C(2)mim][NTf2]) when measured using the same technique. The discrepancies between the p(e) determined in this work for [C(8)mim][NTf2] and [C(2)mim][NTf2] with previous studies is discussed. The enthalpy and entropy of vaporization for all three fluids are estimated from the Clasius-Clapeyron relation. (C) 2016 Elsevier B.V. All rights reserved.