The collisional quenching of vibrationally excited methyl-substituted pyrazine series with an initial energy of 38 000-39 000 cm(-1) by He, Ar, and Kr has been investigated using time-resolved infrared fluorescence of C-H stretching vibrations. The methyl group facilitates the quenching process, but its enhancement effect decreases as the number of methyl groups increases. The quenching efficiency of dimethylpyrazines depends on the position of methyl groups, which becomes significant when going from He to Kr. The steric hindrance of methyl groups plays an important role in the quenching of dimethylpyrazines and trimethylpyrazine by inert gas.