A new series of low-viscosity tetramethylguanidinum-based ionic liquids (ILs) with various substituted phenolate anions were prepared and characterized using nuclear magnetic resonance and Fourier transform irnfrared spectroscopy, elemental analysis, differential scanning calorimetry analysis, and thermogravimetry. Their densities, viscosities, refractive indices, and electrical conductivities were measured and correlated with thermodynamic and empirical equations in the temperature range of (298.15 to 343.15) K under ambient conditions. The effects of substituent groups in phenolate anions on these physical properties were then discussed on the basis of structure. Furthermore, the thermal expansion coefficient was calculated from the experimental values of density, and the correlation between the viscosity and the molar conductivity was proposed by the Walden rule. These ILs are classified as poor ionic liquids and have a poor temperature dependency of the thermal expansion coefficient.