A 3-ethyl-4-methylpyrrole unit in etioporphyrin was shown to be converted to a 3,4-dimethylpyrrole unit by the action of heat, which was proved to be catalyzed by Na-montmorillonite. Time courses of the reaction could be followed by analyzing 2-ethyl-3-methyl- and 2,3-dimethylmaleimide after chromic acid oxidation of the porphyrin. The ratio of these maleimides was proposed as a new maturity indicator for sedimentary organic matter.