This publication describes the first international intercomparison of particle-size determination by single-particle inductively coupled plasma mass spectrometry (sp-ICPMS). Concentrated monodisperse silver nanoparticle suspensions with particle diameters of 20, 40 and 100 nm and a blank solution were sent to 23 laboratories in Europe, the USA and Canada. Laboratories prepared eight nanoparticle preparations in two food simulants (distilled water; 10% ethanol) and reported median particle size, Ag particle mass concentration and Ag particle number concentrations. Average repeatability and reproducibility standard deviation (sr and sR) for the median particle diameter were 1 and 14 nm, respectively. Relative precision was worse for Ag particle number concentrations (RSD r = 11%; RSD R = 78%). While further improvements of the method, especially with respect to software tools for evaluation, hardware options for shorter dwell times, calibration standards for determining nebuliser efficiency and further experience by laboratories are certainly desirable, the results of this study demonstrate the suitability of sp-ICPMS for the detection and quantification of certain kinds of nanoparticles.