The transport and thermal properties of phosphonium ionic liquids (ILs) and thermally stable suspensions of phosphonium ILs and multiwalled carbon nanotubes (MWCNT) were determined. The thermal conductivity was measured over the temperature range T = (280 to 360) K, and at atmospheric pressure, using the transient hot-wire method with an accuracy estimated as 5% from (0.2 to 2) W . m(-1) . K-1 and less than +/- 0.01W . m(-1) K-1 from (0.02 to 0.2) W . m(-1) . K-1. The phosphonium ILs studied were trihexyltetradecylphosphonium phosphinate [(C-6)(3)PC14)][Phosph], tretrabutylmethylphosphonium methylsulfate, [(C-4)(3)PC1)][C1SO4], trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) imide, [(C-6)(3)PC14][NTf2], and trihexyltetradecylphosphonium trispentafluoroethyltrifluorophosphate [(C-6)(3)PC14][FAP]. For all ILs studied, the thermal conductivities were found to lie within the range (0.12 to 0.16) W . m(-1) . K-1. Our measurements combined with selected values from the open literature allowed us to determine a linear increase of combined properties (thermal conductivity, density and molar mass) with the increasing of molar mass. The addition of only (0.1 to 0.2) wt.% of MWCNT to ILs (IoNanofluids) revealed a slight increasing of the thermal conductivity from 0.5% to 1%. There was also observed a slight decrease of the thermal conductivity with increasing temperature for the ILs alone and respective IoNanofluids. The rheological behaviour of [(C-6) 3PC14)][Phosph], and of [(C-6)(3)PC14][NTf2] based IoNanofluids was evaluated at T = (298.15 to 333.15) K and for shear rates in the range gamma = (3 to 30) s(-1). All the samples showed shear thinning behaviour. The thermal stability of the IoNanofluids compared to that of ILs alone was studied by High Resolution Thermogravimetric Analysis. It was found that pure [(C-6)(3)PC14)][Phosph] is a thermally stable IL, whose thermal decomposition under the conditions of this study (2K . min(-1)) and starts above T = 513 K. It exhibits a one step thermogravimetric profile ([(C-6)(3)PC14][NTf2] that was already investigated in a previous study). A small shift towards lower temperatures was observed in the thermogravimetric curves of IoNanofluids compared to those of the respective ILs alone indicating comparable thermal stabilities. The heat capacities determined by Modulated Differential Scanning Calorimetry within a temperature interval ranging from ca. (310 to 515) K with an estimated uncertainty of less than +/- 0.012 J . K-1 . g(-1). (C) 2013 Elsevier Ltd. All rights reserved.