The rate of polymerization is determined by measuring the volüme contraction in dilatometer and from the knowledge of the densities of the monomer and polymer. The method is based on assuming ideal mixing of both the monomer and polymer with the solvent used. In this work, this assumption was tested experimentally in the cases of methyl acrylate-acetone, Polymethyl acrylate-methyl acrylate, polymethyl acrylate-acetone, polymethyl acrylate-methyl acrylate-acetone and methyl acrylate-carbon tetrachloride systems.