According to the current European regulation on cosmetics, any ingredient present as a nanomaterial should be indicated in the ingredient list. There is a need for analytical methods capable of determining the size of the relevant ingredients and thus assessing if these are nanomaterials or not. An analytical method based on asymmetric flow field-flow fractionation (AF4) and inductively coupled plasma-mass spectrometry (ICP-MS) was developed to determine the size of particles present in a commercial toothpaste. Multi-angle light scattering (MALS) was used for on-line size determination. The number-based particle size distributions (PSDs) of the particles were retrieved upon mathematical conversion of the mass-based PSDs recovered from the AF4-ICP-MS fractograms. AF4-ICP-MS allowed to separate and detect Al2O3 and TiO2 particles in the toothpaste and to retrieve a correct TiO2 number-based PSD. The potential presence of particles in the lower size range of the Al2O3 mass-based PSD had a strong impact on sizing and nanomaterial classification upon conversion. AF4 coupled with ICP-MS and MALS was found to be a powerful approach for characterization of different particles in a multiple-particle system such as toothpaste. Confirmation of particle size by a secondary method such as single particle ICP-MS or hydrodynamic chromatography was crucial.