To investigate the formation process of high carbon number (> C-32) sedimentary porphyrins, heating experiments of several porphyrins were performed Chromic acid oxidation of the heating products of protoporphyrin IX dimethyl ester afforded 2-methyl 3 n propylmaleimide as the predominant product among the side-chain extension products formed On the other hand, saturated substituents of etioporphyrin were also extended on heating to slowly form normal and branched homologs These results may suggest that the transalkylation of porphyrin side chains proceeds mainly by a regioselective mechanism involving alkyl radical addition to a vinyl group of chlorophylls or their diagenetic products