Abstract <p>The electron ionization fragmentation of previously unavailable 3,6-substituted 5,6-dihydropyridin-2(1<i>H</i>)-ones synthesized from allenic and acetylenic carbanions, isothiocyanates, and methyl iodide has been studied for the first time. All the studied compounds form the molecular ion <i>M</i>^+•, the fragmentation pathway of which depends on the substituents in both position 3 and position 6. The fragmentation of the molecular ion of 3-methoxy-5,6-dihydropyridin-2(1H)-one is initiated at the radical site, leading to the loss of a radical from position 6 and the localization of the positive charge on the endocyclic nitrogen atom. For 3-phenyl-5,6-dihydropyridin-2(1<i>H</i>)-one, the primary fragmentation pathway involves the formation of an odd-electron ion with <i>m/z</i> 144 (<i>I</i>_rel 100%) via the loss of an HNCO molecule from the <i>M</i>^+• ion or of an MeCH=NH molecule from the isomerized molecular ion <i>M</i>₁^+•. The fragmentation of the molecular ion of 3-methyl-1-azaspiro[5.5]undec-3-en-2-one is characterized by the formation of the radical ions [<i>M</i> – HNCO]^+• (<i>I</i>_rel 100%) and [<i>M</i> – C₄H₈]^+• (<i>I</i>_rel 92%). Experimental evidence has been obtained for the formation of the precursors of 5,6-dihydropyridin-2(1<i>H</i>)-ones―2-(methylsulfanyl)-1,2,5,6-tetrahydropyridin-2-ols, which undergo heteroring cleavage under electron ionization.</p>