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relaxation Abstract: For the mixture carbon disulfide + carbon tetrachloride hypersound, ultrasound speed, and ultrasound damping were measured at 293.15 K.From the experimental data the energy vibrational relaxation time was determined. For the mixture carbon disulfide + carbon tetrachloride Brillouin spectra were measured at 293.15 K and 1 bar. The spectrometer was a Fabry-Perot interferometer with a free spectral range of 15.38 GHz and a finesse of 38 [1]. In addition the speed of sound and sound absorption have been measured, at 293.15 K and at ca. 5, 15, 26 and 36 MHz. Using molar heat capacitites at constant pressure (by flow calorimetry) and molar volumes (by vibrating tube densimetry) from the literature [2], the molar heat capacity at constant volume and the isothermal compressibility have been evaluated. All these date were necessary to extract the energy vibrational relaxation time from the experimental Brillouin spectra. This was done by fitting the experimental Brillouin spectra to theoretical ones, using Mountain`s theory[3] with consideration of the instrumental function of the spectrometer. Figure 1 shows subadditive composition dependences for both, the ultrasound speed and the hypersonic speed, respectively. The sound dispersion is about 95 m/s for pure CS2, and 115 m/s for pure CCl4. Carbon disulfide + Carbon tetrachlorid