A method for determining the average cluster size of supported catalysts using low-energy ion scattering (LEIS) data is presented. The method is particularly suitable for determining the average cluster size of atomically dispersed metals and can be used for clusters as large as 10 nm. For Pt/¿-Al2O3 quantitative agreement is shown between the average cluster size as determined with LEIS and that determined with transmission electron microscopy (TEM). In the case of three-way catalysts, classic methods for determining the cluster size, such as TEM or CO chemisorption, often fail or produce ambiguous results because of the presence of ceria. Therefore, LEIS has been applied to determine the average noble metal cluster size of a commercial three-way catalyst. Moreover, it is shown by LEIS that the three-way catalyst contains mixed Pt/Rh clusters, which are strongly enriched in surface Pt after reduction.