The frequency and the bandwidth of the 12 Raman active lattice modes and of more than 45 internal vibrons of natural and C-12 isotopically pure benzene have been measured under high resolution with the aid of a tandem interferometer-spectrometer system or alternatively of a two meter focal length grating Sopra spectrometer. For five lattice phonons and for several vibrons the measurements were extended to a sufficient temperature range to study the relaxation processes in terms of anharmonic phonon-phonon and phonon-vibron coupling processes. The effect of the isotopic impurities on the phonon lifetime has been shown to be very important for some vibrons, in particular, for those resonant with the impurity level. The bandwidth evolution with temperature has been interpreted in terms of down- and up- three- and four-phonon as well as of dephasing processes.