Ultrasonic dispersion (USD) constitutes an essential part of sample preparation for the characterisation of potential nanomaterials. Corresponding dispersion protocols require a high input of ultrasonic energy into small suspension volumes. The intention behind is to disintegrate agglomerates and aggregates as completely as possible into the constituent particles or at least into smallest dispersible units. However, it is demonstrated that the concept of complete dispersion fails for pyrogenic materials, which are composed of fractal-like aggregates. Moreover, abrasion of the ultrasonic probe sets practical limits on the ultrasonic energy, which should be employed for sample preparation. The paper also discusses strategies to cope with contaminant probe particles. Last but not least, the experimental data support the application of the energy density as the main parameter for comparing sample preparation based on USD. (C) 2017 Elsevier B.V. All rights reserved.