The guanidinium-based ionic liquid (IL), 2,2-diethyl-1,1,3,3-tetramethylguanidinium bis(trifluoromethyl sulfonyl) imide ([TMG(C-2)(2)][NTf2]), was synthesized. Densities and viscosities of the IL and its mixtures with methanol or ethanol were reported at temperatures from 293.15 K to 323.15 K. The increment of IL concentration or the decrement of temperature significantly increases both density and viscosity. Excess molar volume (V-m(E)) and viscosity deviation (Delta eta) of each binary mixture were calculated, and the Redlich-Kister equation was used to fit V-m(E) and Delta eta with composition. When V-m(E) is negative with the mole fraction of IL (x(1)) lower than 0.8 for two systems, Delta eta is negative over the whole concentration range. The positive V-m(E) of the mixture at a high IL concentration indicates that interaction loss occurs. Moreover, structures of the equimolar mixture of IL and alcohol were obtained by DFT calculations, and alcohol-anion, alcohol-cation and cation-anion interactions were revealed. Strong H-bond interaction with the span of 1.61 angstrom is formed between the H atom of the -OH group in alcohol and the O atom in the [NTf2] anion, while the O atom of the -OH group in alcohol interacts with the methyl groups in the [TMG(C-2)(2)](+) cation. Significant cation-anion interactions (<2.40 angstrom) in the manners of H center dot center dot center dot F and H center dot center dot center dot O with spans less than 2.40 angstrom are found at the same time. It is the enhanced H-bond interactions that contribute to the negative V-m(E) of the equimolar mixture at non-concentrated regions. (C) 2019 Elsevier Ltd.