Collisional quenching of vibrationally excited methyl-substituted pyrazine and pyridine molecules by CO2 has been investigated using the infrared emission of C-H stretching vibrations in gas phase. The methyl substitution increases the collisional quenching efficiency, but its enhancement effect decreases as the number of methyl groups increases. The pyrazine series are on average more efficiently quenched compared to the pyridine series. The quenching efficiency of dimethylpyrazine and dimethylpyridine isomers depends on the position of two methyl groups. We suggest that the collision pathways involving the ring nitrogen atom may be the efficient quenching channels.