Environmentally friendly, biodegradable biopolymers can be prepared via the chemical modification of starch. These biopolymers are able to partially or totally substitute synthetic additives used today in the chemical or environmental technologies. Through the building of ionic function groups into polymeric chains of the starch ion exchangers, flocculants or dispersants can be produced. In this paper, we examine water-soluble ionic derivatives called polyelectrolytes. By phosphorylating starch while preserving or maybe increasing the molecular weight of the native starch polymers, anionic flocculating agents can be prepared. If the polymer chains degrade during the reaction, the products will act as dispersing agents in aqueous suspensions. It is apparent that, during the phosphorylation reaction, the changes in the molecular weight distribution and the ionic charge of the polymers have crucial importance. In our experiments, we investigated the solid-phase reactions by following the change in the molecular weight distributions, using size exclusion chromatography, and the change in the charge densities, using a particle charge detector. The efficiency of the products was tested in the laboratory, and the results were evaluated by means of severity parameters.