An aqueous dispersion of poly (acrylic acid)-stabilised cerium oxide (CeO 2 ) nanoparticles (PAA-CeO 2 ) was evaluated for its stability in a range of freshwater ecotoxicity media (MHRW, TG 201 and M7), with and without natural organic matter (NOM). In a 15day dispersion stability study, PAA-CeO 2 did not undergo significant aggregation in any media type. Zeta potential varied between media types and was influenced by PAA-CeO 2 concentration, but remained constant over 15days. NOM had no influence on PAA-CeO 2 aggregation or zeta potential. The ecotoxicity of the PAA-CeO 2 dispersion was investigated in 72h algal growth inhibition tests using the freshwater microalgae Pseudokirchneriella subcapitata. PAA-CeO 2 EC 50 values for growth inhibition (GI; 0.024mg/L) were 2–3 orders of magnitude lower than pristine CeO 2 EC 50 values reported in the literature. The concentration of dissolved cerium (Ce 3+ /Ce 4+ ) in PAA-CeO 2 exposure suspensions was very low, ranging between 0.5 and 5.6μg/L. Free PAA concentration in the exposure solutions (0.0096–0.0384mg/L) was significantly lower than the EC 10 growth inhibition (47.7mg/L) value of pure PAA, indicating that free PAA did not contribute to the observed toxicity. Elemental analysis indicated that up to 38% of the total Cerium becomes directly associated with the algal cells during the 72h exposure. TOF-SIMS analysis of algal cell wall compounds indicated three different modes of action, including a significant oxidative stress response to PAA-CeO 2 exposure. In contrast to pristine CeO 2 nanoparticles, which rapidly aggregate in standard ecotoxicity media, PAA-stabilised CeO 2 nanoparticles remain dispersed and available to water column species. Interaction of PAA with cell wall components, which could be responsible for the observed biomarker alterations, could not be excluded. This study indicates that the increased dispersion stability of PAA-CeO 2 leads to an increase in toxicity compared to pristine non-stabilised forms.