A mass spectral fragmentation study for a number of NN-linked bi(pyridones) (V-VIII) has been carried out. The effect of substituents at the 2-, 4- and 6-position for these heterocycles have been investigated. The isomeric structures. VIII and IX might reasonably be exp"~ted to exhibit i.r: and n.m.r. spectra bearing some similarity to each other. The differentiation between the isomeric structures VIII and IX was achieved by mass spectrometry. Whereas the molecular ions (M'+) give frag­ment ions (M'+ -CO) due to the loss oftheC=O groups at the 2-and4-positions, daughter iOlls R-C=N'+ and R-O=O+ are obtained for 6-substituted pyridones and pyrans respectively.