Etioporphyrin, one of the most common sedimentary porphyrins, was heated at 300 to 450 degrees C to investigate the diagenetic formation of benzoporphyrins. Chromic acid oxidation of the resulting products afforded maleimides, as well as phthalimides, indicating the formation of benzopyrrole units in the substrate. Isomer ratios of dimethylphthalimides formed during heating were in accordance with those obtained by chromic acid oxidation of the Cretaceous/Tertiary sediments at Rumoi, Hokkaido, which suggests that alkylporphyrins could be transformed into benzoporphyrins in sediments.