We detected a homologous series of phthalimides in organic solvent extracts of Cretaceous to Quaternary sediments. 2-Ethyl-3-methylmaleimide has been found in present-day sediments as a degradation product of chlorophylls; therefore, an authentic sample was heated, affording phthalimide and its homologs as well as homologs of maleimide. The heating products showed a similar molecular distribution of phthalimide and methylphthalimides to those found in the sediments analyzed. These results suggest that the phthalimides found in old sediments are derived from chlorophylls via 2-ethyl-3-methylmaleimide by geochemical transformation.