Agglomeration and disagglomeration processes are expected to play a key role on the fate of engineered nanoparticles in natural aquatic systems. These processes are investigated here in detail by studying first the stability of TiO2 nanoparticles in the presence of monovalent and divalent electrolytes at different pHs (below and above the point of zero charge of TiO2) and discussing the importance of specific divalent cation adsorption with the help of the DLVO theory as well as the importance of the nature of the counterions. Then the impact of one polysaccharide (alginate) on the stability of agglomerates formed under pH and water hardness representative of Lake Geneva environmental conditions is investigated. In these conditions the large TiO2 agglomerates (diameter N 1 μm) are positively charged due to Ca2+ and Mg2+ specific adsorption and alginate, which is negatively charged, adsorbs onto the agglomerate surface. Our results indicate that the presence of alginate at typical natural organic matter concentration (1–10 mg L−1) strongly modifies the TiO2 agglomerate (50 mg L−1) stability by inducing their partial and rapid disagglomeration. The importance of disagglomeration is found dependent on the alginate concentrationwithmaximumof disagglomeration obtained for alginate concentration ≥8mg L−1 and leading to 400 nmfragments. Froman environmental point of viewpartial restabilization of TiO2 agglomerates in the presence of alginate constitutes an important outcome. Disagglomerationwill enhance their transport and residence time in aquatic systems which is an important step in the current knowledge on risk assessment associated to engineered nanoparticles.