The chemical changes occurring in an olive oil mill wastewater (OMW) sample digested catalytically with MnO(2) for 30 and 60 days were evaluated comparatively with those occurring in the same OMW left standing for the same time in an open-air lagoon. Both treatments increased the pH and electrical conductivity and decreased the contents of dry matter, total organic C and total N, and C/N ratio of OMW. The humic acid (HA)-like fraction isolated from the fresh OMW was characterized by a marked aliphatic character, small O and acidic functional group contents, marked presence of proteinaceous materials, partially modified lignin moieties and polysaccharides-like structures, extended molecular heterogeneity, and small degrees of aromatic ring polycondensation, polymerization and humification. With increasing the time of either lagooning or catalytic digestion, a loss of aliphatic materials and an increase of extraction yield, oxygenation, acidic functional groups, carbohydrates and aromaticity occurred in the HA-like fractions. The more evident changes measured for the HA-like fractions from catalytically-digested OMW, with respect to those from lagooned OMW, indicated that MnO(2) was able to catalyze organic matter humification in OMW. (c) 2008 Elsevier Ltd. All rights reserved.