The stabilities of the charge transfer complexes of pentamethyl benzene with iodine in n-hexane have been investigated by UV-spectrophotometric measurements at 25, 40 and 60$^{circ}C$ up to 1600 bars. The equilibrium constant of the complex formation was increased with pressure while being decreased with temperature raising. Changes of volume, enthalpy, free energy and entropy for the formation of the complexes were obtained from the equilibrium constants. The red-shift at higher pressure, the blue-shift at higher temperature, and the relation between pressure and oscillator strength have been discussed by means of thermodynamic functions. In comparison with the results in the previous studies, the absolute values of ${Delta}$V at each temperature were increased with the number of methyl groups of polymethyl benzene. However, it can be seen that both ${Delta}$H and ${Delta}$S show extreme behaviors in durene near atmospheric pressure but they are negatively increased with the number of methyl groups near 1600 bar. This order of the thermodynamic parameters may be a measure of the relative basicities of polymethyl benzenes toward iodine under each pressure, and these phenomena are explained in terms of a positive inductive effect and a steric hindrance effect of the polymethyl benzene molecule.