The electrical conductivity, density and diffusion coefficients of trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P66614][NTf₂]) ionic liquid and its binary solutions in acetonitrile, propionitrile, dimethyl and diethyl carbonates were measured in the temperature range of 293–348 K. The electrical conductivity – ionic liquid mole fraction dependencies for the binary solutions were fitted with the empirical Casteel–Amis equation. The temperature dependencies of electrical conductivity were analyzed using the Arrhenius, Litovitz and Vogel–Fulcher–Tammann approaches. The dependences of the Arrhenius activation energy and pre-exponential factor on the mole fraction of ionic liquid in the solutions were fitted with the empirical equations proposed in the literature. The thermo-gravimetric analysis combined with mass spectrometry demonstrated the high thermal stability of [P66614][NTf₂] up to 600 K. At higher temperatures the decomposition of [P66614][NTf₂] proceeded via the elimination of alkyl radicals as a result of the nucleophilic attack of reactive intermediates to the [P66614]⁺ cation with the formation of trialkylphosphines. The activation energies of the thermal destruction of [P66614][NTf₂] were calculated using the Kissinger equation and non-linear integral isoconversional model.