The femtosecond optically heterodyned detected optical Kerr effect (OHD-OKE) response of carbon disulfide/ pentane binary mixtures is reported for various concentrations. As pentane is added, three trends are observed : the instantaneous electronic contribution gets more pronounced, the picosecond diffusive reorientation becomes faster, and the subpicosecond inertia-limited response slows down. The corresponding frequency domain representation of the latter component narrows and shifts toward lower frequencies. These spectral changes occur exclusively when the CS2 concentration is lowered from 100 to 20 vol %; at still lower concentrations no further changes are observed. When the nondiffusive part of the optical response is assigned exclusively to coherent intermolecular vibrations, the spectral changes can be attributed to a reduced inhomogeneous broadening, reflecting a narrowing of the distribution of oscillator frequencies. However, we argue here that because of the observed concentration dependence the validity of this model is questionable. Long-range interaction-induced effects and short-range collision-induced processes are additional scattering mechanisms that may contribute substantially to the observed spectra.