Chitosans of different molecular weights were assessed as cement admixtures to modify the rheological properties of a fresh cement paste. Their sequestering capability with respect to several heavy and toxic metals was checked and the complexation processes were investigated by different approaches based on electrochemical measurements. The aim was to conduct research on a potentially useful polymer to be added to cementitious materials acting as a viscosity modifier as well as a heavy metal immobilizing agent. Pb, Cu, Zn, Cd, Mo, and Cr were used as target metals. The highest complexation affinity was found toward Zn, with stoichiometries up to 1 : 40 having been calculated. Chitosan was proved to act as a thickener in cement mixtures. The highest molecular weight chitosan showed the greatest effectiveness taking into account its combined role as heavy metal retainer and thickener. In addition, a clear interaction between this high molecular weight chitosan and cement particles was also demonstrated by means of zeta potential measurements. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 242252, 2011