In this work, a series of imidazolium-based ionic liquids with varying functionalities from aliphatic to aromatic groups and a fixed anion, bis[(trifluoromethane)-sulfonyl]amide, were investigated. The imidazolium cations included 1-heptyl-3-methylimidazolium, 1-(cyclohexylmethyl)-3-methylimidazolium, 1-benzyl-3-methylimidazolium, 1,3-dibenzylimidazolium, and 1-(2-naphthylmethyl)-3-methylimidazolium. Structure-property relationships were investigated regarding the substituent effects on the imidazolium cation, including n-alkyl versus cycloalkyl and aromatic versus aliphatic, as well as the effects of cation symmetry and larger aromatic polycyclic functionalities. Thermophysical properties investigated include density, thermal transition temperatures, and decomposition temperatures. The densities of the ionic liquids are governed by the substituents on the cation: N-alkyl < cycloalkyl < aromatic. The group contribution method is applicable for the density estimation of ionic liquids, and the volume parameters for cyclohexyl, phenyl, and naphthyl groups are reported. The glass transition temperature (Tg) follows the same systematic trend due to substituent flexibility: N-alkyl < cycloalkyl < aromatic. Thermal stability as measured by dynamic thermogravimetric analysis (TGA) is not strongly affected by the substituents on the imidazolium ring; however, slight differences are observed with the higher Tg ionic liquids having lower decomposition temperatures for this series of ionic liquids. On the other hand, the cyclohexylmethyl-substituted ionic liquid exhibits a higher activation energy for degradation than the other ionic liquids based on isothermal TGA, and all ionic liquids studied show significant weight loss at 300 °C indicating that appreciable decomposition can occur at temperatures substantially lower than the onset temperature observed in dynamic TGA scans.