The chemical changes occurring in a cattle manure (CM) and a mixture of two-phase olive pomace and CM (OP + CM) after vermicomposting with Eisenia andrei for eight months were evaluated. Further, humic acid (HA)-like fractions were isolated from the two substrates before and after the vermicomposting process, and analyzed for elemental and acidic functional group composition, and by ultraviolet/visible, Fourier transform infrared and fluorescence spectroscopies. Before vermicomposting, the HA-like fractions featured a prevalent aliphatic character, large C contents, small O and acidic functional group contents, a marked presence of protein-accous materials and polysaccharide-like structures, extended molecular heterogeneity and small degrees of aromatic ring polycondensation, polymerisation and humification. After vermicomposting, the total extractable C and HA-C contents in the bulk substrates increased, and the C and H contents, aliphatic structures, polypeptidic components and carbohydrates decreased in the HA-like fractions, whereas O and acidic functional group contents increased. Further, an adequate degree of maturity and stability was achieved after vermincomposting, and the HA-like fractions, especially that from OP + CM, approached the characteristics typical of native soil HA. Vermicomposting was thus able to promote organic matter humification in both CM alone and in the mixture OP + CM, thus enhancing the quality of these materials as soil organic amendments. (C) 2007 Elsevier Ltd. All rights reserved.