In this work, the effect of SO2 on the densities and viscosities during absorption processes of pyridinium-based ionic liquids (ILs), such as the conventional ILs [C4Py][BF4] and [C4Py][SCN] and the tertiary amino-functionalized IL [NEt2C2Py][SCN], was investigated. The mechanism of the variations in these two physical properties was also explained in detail through a combination of experimental methods and simulation calculations. The results indicated that the densities of the conventional and functionalized ILs increased gradually with increasing amounts of SO2. However, the viscosities of two kinds of ILs during SO2 absorption showed different variation trends according to different mechanisms of SO2 absorption, i.e., physical absorption and chemical absorption, which was proven by the in situ Fourier transform infrared results. The viscosity changes of the functionalized IL [NEt2C2Py][SCN] during SO2 absorption experienced two stages. First, the viscosity increased sharply because of chemical interac...