We report the preliminary results of direct extraction of complex permittivity of alginate nanoparticles solutions using microwave open-ended coaxial probing. The study focused on identifying main technical obstacles in such measurements – especially the influence of concentration and aggregation of the nanoparticles on the measured dielectric constant and loss tangent. The experimental data revealed that an increase of alginate nanoparticles concentration is correlated with changes of both loss tangent and dielectric constant.