Concentration dependences of the isotropic Raman line widths for the $ { u}_1$ (C-H and C-D) and the ${ u}_2;(C{equiv}N)$ modes for $CH_3CN$ and $CD_3CN;in;CCl_4$ solutions have been investigated. For the ${ u}_1$ modes, the dephasing seems to be a good description for the isotropic line broadening in $CCl_4$ solution even though the concentration fluctuation can reproduce the experimental results also. For the ${ u}_2$ modes, the concentration fluctuation was the dominant mechanism for the concentration-dependent line broadening.