A series of ammonium-based ionic liquids (ILs), which share a homologous series of cations (CH3CH2)(3)N+(CnH2n+1) with n = 2, 4, 6, 8 and the anions with either BF4-, PF6-, or SbF6-, was synthesized. Their structures were confirmed by H-1 and C-13 NMR, ESI-MS, and elemental analysis. Meanwhile, the content of impurity (e.g., water and bromide ions) was also determined using Karl Fischer titrator and ion chromatography. The thermal properties of the ILs were determined by TGA and DSC. Five of the investigated ILs have been shown to have a low melting point (< 100 degrees C): N,N,N,N-tetraethylammonium tetrafluoroborate, [N-2222]BF4, N,N,N,N-tetraethylammonium hexafluorophosphate, [N-2222]PF6, N,N,N-triethylhexylammonium tetrafluoroborate, [N-2226]BF4, N,N,N-triethyloctylammonium hexafluorophosphate, [N-2228]PF6 and N,N,N-triethyloctylammonium hexafluoroantimonate, [N-2228]SbF6. Densities, refractive indices, and miscibility of these 12 ILs were well studied systematically. Moreover, from the analysis of the structure-property relationship, the role of the alkyl chain length of the cation on these physical properties of the ILs has been assessed, and the influence of the nature of the anions on these experimental data of the ILs has been discussed. The studies may provide valuable contributions for the design and study of ILs.