The present work is an attempt to obtain information on the rheological behaviour of sewage sludge by performing electrical resistivity measurements. The underlying hypothesis is that, in such gel-like complex fluids, the electrical conductivity is determined not only by the free ions charge and mobility in the interstitial solution, like in simple electrolyte solutions, but also by the structure and the dynamics of the macromolecular and colloidal network. In addition, electrical measurements offer the advantage of being a non-destructive technique suitable for implementation in the field. The viscosity and resistivity of sludges containing between 4 and 15% dry matter were measured as a function of three parameters: (i) temperature, from 278 to 323 K, (ii) added salt concentration, between 0 and 22.3% KCl with respect to dry mass and (iii) ageing, from 0 to 25 days. Both viscosity and resistivity were found to follow the same non-Arrhenius temperature dependence, typical of cooperative molecular motions in glass-forming liquids, with the same thermodynamic parameters. In spite of this complex behaviour, a linear relationship between viscosity and resistivity was obtained. This linear relationship was destroyed by the addition of salt and by ageing, but an empirical parameter combining resistivity and viscosity could still be linearly related to resistivity, allowing for the indirect determination of the sludge viscosity from electrical measurements. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.