In this study, results about sludge stabilization in reed bed systems (RBSs) after the entire period of operation in two different systems situated in Denmark (Helsinge 42,000 population equivalent (p.e.) - 10 years) and in Italy (La Fontina, 30,000 p.e. - 6 years) were presented. In order to evaluate the process of sludge stabilization, parameters that highlighted the biochemical and chemico-structural properties of organic sludge matter have been determined. The level of total and soluble nutrients, and enzyme activities, parameters related to overall microbial activity, showed that stabilization of the sludge similarly occurred in both RBSs, even though in different landscape ecosystems. The chemical-structural characterization of sludge organic matter highlighted how the processes of stabilization have occurred satisfactorily in both RBSs; in fact, significant levels of pyrolytic indices for mineralization and humification were reached. The successful stabilization of organic matter occurred in both RBSs and was confirmed by the absence of Escherichia coli, and also by the results of organic compounds (polycyclic aromatic hydrocarbons, linear alkyl benzene sulfonates, nonylphenol ethoxylates, di-2-ethylhexyl-phthalate) and heavy metals.