The density and surface tension of 1 methyl-3 methylimidazolium dimethylphosphate [C(1)mim] (CH3O)(2)PO2 and 1-ethyl-3-methylimidazolium diethylphosphate [C(2)mim](CH3CH2O)(2)PO2 ionic liquids have been measured over the temperature range from (283 15 to 338 15) K The coefficients of thermal expansion were calculated from the experimental density results using an empirical correlation for T = (283 15 to 338 15) K Molecular volume and standard entropies of these ILs were calculated from the experimental density values The surface properties of ILs were investigated The critical temperature and enthalpy of vapourization were also discussed (C) 2010 Elsevier Ltd All rights reserved