In spite of the great importance of the volumetric, transport, and surface properties of phosphonium-based ionic liquids, only limited information on these values seems to be available in the open literature. In this work, we present the results for the density measurements of the trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate, [(C6)3PC14][(C2F5)3PF3, FAP] over the ranges of temperature and pressure, T=(293.15 to 343.15)K and p=(0.1 to 25.0)MPa, respectively with an estimated uncertainty of ±0.5kg·m−3. The viscosity and the surface tension were measured over the ranges T=(283.10 to 363.17)K and T=(298.51 to 343.29)K with maximum uncertainties of ±2.0% and ±0.4mN·m−1. The experimental results on density were correlated using the Goharshadi–Morsali–Abbaspour equation of state. From this equation, thermomechanical coefficients as the isothermal compressibility, thermal expansivity, thermal pressure, and internal pressure were calculated. The Sanchez–Lacombe equation of state was also used for (pVT) correlation and the estimation of the free volume of [(C6)3PC14][FAP] which has been compared with other [(C6)3PC14]-based ionic liquids with variable anions. The Vogel–Fulcher–Tammann (VFT) equation for viscosity was used to correlate the viscosity. From the analysis of viscosity results for [(C6)3PC14]-based ionic liquids, a new prediction method has been developed for this property. Using the linear variation of the surface tension with the temperature, the surface entropy and enthalpy of [(C6)3PC14][FAP] were found. The reciprocal of dynamic viscosity (or fluidity) was correlated with temperature and a new relation between fluidity and surface tension was tested to [(C6)3PC14] ionic liquids.