We have studied by high resolution infrared spectroscopy the v(3) vibron band in natural and isotopically pure S-32 sulfur crystals. From the comparison of the spectra we have assigned the bands due to-the different molecules containing one or two atoms of S-34 as well as one atom of S-33. We have measured tl;e bandwidth and the frequency of the vibron as a function of temperature from 10 to 140 K both in the natural and in the isotopically pure crystal and we have interpreted the results in terms of decay and scattering processes. In particular we have explained the nonlinear evolution of the bandwidth and of the frequency shifts with temperature in terms of scattering processes controlled by quartic terms of the crystal Hamiltonian.