The effect of pressure and temperature on the stabilities of the charge transfer complexes of 1,3,5-trinitrobenzene, tetrachloro-p-benzoquinone and tetracyanoethylene with hexamethylbenzene in carbon tetrachloride has been investigated by spectrophotometric measurements. The equilibrium constants for the formation of the complexes were obtained at various temperature and pressure, and thermodynamic parameters for the formation of the charge transfer complexes are calculated from these values. The relative stabilities of charge transfer complexes with hexamethylbenzene increase in the order; 1,3,5-trinitrobenzene < tetrachloro-p-benzoquinone < tetracyanoethylene. This may be regarded as an order of relative acidity of these compounds in complexation with hexamethylbenzene and is explained in terms of the negative inductive effect of the <TEX>${pi}$<TEX> acceptors. The red-shift at higher pressure, the blue-shift at higher temperature and the relation between pressure and oscillator strength are discussed on the basis of thermodynamic functions.