Heating of chlorophyll a was performed at 350 degreesC for 12 h in relation to the origin of sedimentary benzoporphyrins. The product was oxidized by chromic acid to give phthalimide and its methyl homologs, which indicated the formation of benzopyrrole units in the substrate or its degraded fragments. The heating also produced a monomethylpyrrole unit. suggesting that a vinyl group of the chlorophyll was used as a part of the thermally generated benzopyrrole units.