3-Formyl- and 3-acetylpyridine N(4)-methylthiosemicarbazone, H3Fo4M and H3Ac4M, respectively, have been prepared, their structures solved and IR, NMR and UV spectra recorded. Intramolecular and intermolecular hydrogen bonding are both present, but the mode of the intermolecular interaction is different in the two compounds. Further, there is a significant difference in their angles between the mean planes of the pyridine ring and thiosemicarbazone moiety. The 1H NMR spectral studies show significant differences between the two.