The synthesis and characterization of a series of new phenyl/alkyl-substituted-3,5-dimethylpyrazolium ionic liquids ([Phpz(R)][X] R = CnH2n+1 n: 1,2,3,4,5,6,7 [X] = CH3SO3-, BF4-, PF6-) are described. Their melting points, thermal stabilities, electrochemical windows, and solubility properties in common solvents were investigated. They were found to exhibit very good electrochemical and thermal stabilities. The results indicate that these ionic liquids have a high thermal stability up to 374 degrees C and a large electrochemical window of 4.63 V. The thermophysical properties such as density, viscosity and refractive index were also measured as a function of temperature for the ionic liquids which are in liquid state at room temperature. (C) 2014 Elsevier B.V. All rights reserved.