The Mannich bases derived from phenylacetone (I, R=Me) and dibenzyl ketone (I, R=CH₂Ph) prepared in our previous work⁽¹⁾ gave expected semicarba-zones (Ⅱ) in neutral or slightly acidic media. Degradation of their methiodides gave vinyl ketone semicarbazones (Ⅲ). The structure of the latter was shown by their characteristic ultra-violet light absorption properties, and they gave intense absorption bands near 260 mμ⁽³⁾.