Long-term stability of two engineered nanomaterials (ENMs), i.e., the inorganic n-TiO2 and the organic Multi-Walled Carbon Nanotubes (MWCNTs), dispersed in artificial freshwater (5–100 mg l-1), was investigated from short-term settling velocity, particle size distribution, and surface charge. Hydrodynamic diameter and f-pot, calculated by means of dynamic and electrophoretic light scattering, respectively, qualitatively indicated a general ENMs dispersion instability over 1 h time. Sedimentation results, obtained by centrifugal separation analysis using the LUMiSizer over approx. 30 min analysis time, allowed to estimate the quantitative long-term (over 30 days) stability of ENMs. Settling data fitted satisfactorily with a first-order kinetic equation (R2 in the range of 0.918–0.989). The settling rate constant k values extrapolated at gravity spanned one order of magnitude, i.e., from 7.21 9 10-5 to 4.12 9 10-4 s-1, and with the increasing of initial ENMs concentration. Sedimentation velocities were in good agreement with short- to long-term literature data (7.8 9 10-2–1.7 9 10-1 m day-1 vs. 5 9 10-4–3 9 10-1 m day-1 for n-TiO2 and 5.9 9 10-2–3.4 9 10-1 m day-1 vs. 2 9 10-1–1.2 m day-1 for MWCNTs). n-TiO2 showed a higher longterm stability with respect to MWCNTs (average: 1 9 10-1 ± 3.4 9 10-2 m day-1 instead of 1.7 9 10-1 ± 1.1 9 10-1 m day-1, respectively).