Very dilute acqueous pseudo-solutions of a polielectrolite organic polymer with a considerably larger viscosity than pure water are submitted to free cylindrical convection in the region of 4°C, where such solutions still present a density maximum close to that of the pure solvent. Effects of increasing viscosity on the peculiar arrests in temperature vs time curves are observed and interpreted in terms of a theoretical model of convection, coupled with the substantially viscoelastic behaviour of the colloidal solutions.