In spite of the great importance of the (P, V. T) data of phosphonium-based ionic liquids, only limited information on these data seems to be available in the open literature. In this work, we present the results for the density measurements of the trihexyltetradecylphosphonium chloride, [(C6H13)(3)P(C14H29)][Cl] and trihexyltetradecylphosphonium dicyanamide, [(C6H13)(3)P(C14H29)][N(CN2)] with an estimated uncertainty of +/- 0.5 kg.m(-3). The ranges of temperature and pressure are T = (273.15 to 318.15) K and p = (0.1 to 25) MPa for [(C6H13)(3)P(C14H29)][Cl] and T = (273.15 to 318.15) K and p = (0.1 to 35) MPa for [(C6H13)(3)P(C14H29)][N(CN2)]. The high consistency of Our data for [(C6H13)(3)P(C14H29)][Cl] compared with those measured by other authors allowed all the experimental data for this IL to be combined and correlated using the Goharshadi-Morsali-Abbaspour equation of state over a wide range of temperature and pressure. From this equation, thermomechanical coefficients as the isothermal compressibility, thermal expansivity, thermal pressure, and internal pressure were calculated for the two ILs. The Sanchez-Lacombe equation of state was used also for (P, V, T) correlation and the estimation of the free volume in these phosphonium ionic liquids. Finally ionic volumes for trihexyltetradecylphosphonium cation and several anions available in the literature made possible the calculation of the free (hole) volume. (c) 2011 Elsevier Ltd. All rights reserved.