Thermodynamic properties including excess volumes, viscosities and excess Gibbs activation energies of viscous flow were measured for two deep eutectic solvents (DESs) mixed with toluene over the entire composition range. Two DESs were formed by combining tetrabutylphosphonium bromide and levulinic acid (TBPBr:Lev) or tetrabutylammonium bromide and decanoic acid (TBABr:Dec). The DESs were assumed to behave as a pseudo-pure component. The excess volumes values for TBABr:Dec, were negative throughout the composition range, whereas TBPBr:Lev showed negative values only for DES mole fractions higher than 0.2. The excess Gibbs energies of activation of viscous flow were found to be positive for the TBABr:Dec, and changing from positive to negative for TBPBr:Lev around 0.4 mol fraction. These values remain smaller than RT, the energy of the thermal motion of the components of the mixture. Deviations from the averaged viscosity of the pure components were negative for both mixtures across all compositions. The analysis of the IR and FT-Raman spectra showed minimal influence of the mixture composition on vibration modes. Quantum calculations were used to assign the vibration modes. Our results suggest that the variation in thermodynamic data with composition arises from the packing arrangement of the mixture components.