Synthetic water-soluble meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinates of Mn(III) chloride [Mn-(TDCPPS)Cl] and iron(III) chloride [Fe-(TDCPPS)Cl] were employed as biomimetic catalysts in the photooxidative coupling of a terrestrial humic acid (HA). The changes in molecular dimension of HA induced by irradiation with UV light for different periods were followed by high-performance size-exclusion chromatography (HPSEC), with both spectrophotometric and refractive index (RI) detectors. Enhancement of apparent weight-average molecular weight (M-wa) of sodium humate solutions at both pH 7 and 3.5, occurred after irradiation with UV light after 5.5 and 13 h by catalysis with either manganese or iron porphyrin. A significant increase in M-wa values was also found when samples were kept in the dark for 8 and I I days after the end of irradiation, thereby suggesting a progression of free-radicals coupling with time course after photoinduction. The enhancement of absorptivity of humic matter with catalyzed photooxidation was confirmed by the significant increase in apparent molecular mass as shown by the RI detector. The latter also indicated a slightly larger effect by iron-porphyrin than by manganese-porphyrin in the photooxidative coupling of humic molecules. These findings suggest that photooxidation may represent a valid alternative to oxidizing agents, such as H2O2, in the polymerization of natural organic matter catalyzed by biomimetic metalporphyrins.