The infrared spectra of pyrazine, its eight mono-substituted derivatives, and their ten N-oxide derivatives were analyzed. Their characteristic N-O streching frequencies (a strong absorption in the region of 1250 and 1350 cm^-1) were found to be correlated to the nature of a substituent present and the linear relationship between the N-O frequency and the σ-value of a substituent in substituted pyridine N-oxides could be extended to that in substituted pyrazine N-oxides by applying the σ-value of ring nitrogen (0.93) and the N-oxide group (0.25) toward 4-position of pyridine for mono- and di-N-oxides, respectively. The considerably higher frequency shift of the N-O frequency was shown to be characteristic for the methyl group at β-position with respect to the N-oxide group. Ring and ring CH vibrations were discussed on the basis of the correlations found in the pyridine ring system. The splitting of the carbonyl stretching frequency of ester group adjacent to ring nitrogen was pointed out and discussed. The infrared spectra of pyrimidine N-oxide and its eight derivatives were also examined and discussed in comparison with the above results.