In this article composting of 40% Posidonia oceanica, 20% ligno-cellulosic residues, and 40% sewage sludge was monitored by retrieving samples every 2 weeks from the composting pile and by characterizing them both by chemical and spectroscopic analysis in Fourier transform infrared spectroscopy and fluorescence spectroscopy in the emission mode and as EEM spectra. Chemical data indicated the occurrence of extended mineralization and stabilization during the process. The decreasing trend of E 4 /E 6 ratios suggested a progressive humification of the substrate. Ratios of peaks of aliphatics to aromatics as a function of composting time showed a decreasing trend due to consumption of aliphatic structures and increase of aromatics during the process. The trend of fluorescence intensity observed on emission spectra showed an increase up until 6 weeks and then a decrease until the end of the process, as a consequence of the decomposition process, which broke down organic macromolecules contained in the fresh substrate followed by the humification process. EEM spectra of all samples produced a peak D in the range 345-355 ex /437-446 em . Samples retrieved at the beginning of the process showed, in addition, a peak B at 280 ex /344 em whose disappearance during the process was consistent with bio-degradation of organic matter. Samples retrieved at the beginning and after 2, 4, 6, 8, 10, and 12 weeks exhibited also a peak C at 255 ex /440 em whose fluorescence intensity trend suggested a progressive humification of the substrates.Correspondence to: