Two hydrophobic ionic liquids (ILs) 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C(2)mim][NTf2] and [C(4)mim][NTf2] were synthesized. The binary mixtures of the two ILs were prepared with dimethyl carbonate (DMC) or diethyl carbonate (DEC) over the entire mole fraction range, respectively. The density and viscosity of the binary mixtures were determined by an automated DMA 5000 M and a Lovis 2000 ME Anton Paar Rotational Stabinger Viscometer at temperatures ranging from 288.15 to 328.15 K. The uncertainties of the density and viscosity are u(r)(rho) = 0.00120 and u(r)(eta eta) = 0.01. The excess molar volumes and viscosity deviations were calculated for the four binary mixtures by the measurement values. The effects of the extension of the alkyl chain of the cation and anion on the above properties are discussed. The effects of the methyl introduction on the cation was also discussed with the literature. The excess molar volumes and viscosity deviations were fitted versus mole fraction according to the Redlich-Kister equation. The thermal expansion coefficients of the systems were calculated according to the density values.