Synthesis and characterization of a new cement-based polymer nanocomposite is reported. Calcium silicate hydrate (C-S-H) was prepared in the presence of aniline monomer followed by in situ polymerization to increase the degree of interaction between inorganic and organic phases. Two stoichiometrically different C-S-H systems were used. The properties of the C-S-H/polyaniline materials were studied using several analytical techniques including SEM, XRD, TGA, Si-29 MAS NMR and FTIR. It is suggested that the in situ polymerization method can effectively be employed for producing a C-S-H/Polymer nanocomposite. The extent of molecular interaction with the polymer depends on the chemical composition of the C-S-H. Production of a new range of polymer-modified cement-based systems having improved environmental stability and mechanical performance is promising.