In this study, an original method of characterization of water/solid matter interactions in sewage sludge has been developed, based on both rheological characteristics and thermodynamic water activity determinations, in order to check whether a link could be made with water activity and its mechanical properties. The effect of solid matter content, flocculation and ageing time on water activity and rheological parameters has been investigated. Through this study, we showed that rheological parameters (G' and G") of both raw and flocculated sewage sludge at optimal dose of polymer increase with solids concentration following a power-law, whereas in the same way water activity decreases following an exponential relationship (Arrhenius dependence). A slight increase of water activity values and those of a and G" moduli with added polymer was also highlighted. On the other hand, we have shown that during ageing, the rheological parameters G' and G" decreased upon increasing the ageing time whereas in the same way, water activity increases. Rheological parameters clearly evidenced thermodynamic water activity dependence, regarding solid concentration and ageing time, with a decrease of G' and G" upon increasing water activity. Thus, water/solid matter interactions were supposed to intervene importantly in the rheology of sewage sludge. From all these results, it appears clearly that water activity measurements can be regarded as an effective and easy tool for evaluating the state of structuration of sewage sludge and for predicting its structural, textural and mechanical properties during dewatering and ageing. (C) 2013 Elsevier B.V. All rights reserved.