This file contains the Supporting Information associated with the publication “Interlaboratory comparison reveals state-of-the-art in microplastic detection and quantification methods”. Anal. Chem. 2025, 97, 16, 8719–8728 The document provides detailed supplementary material supporting the results of a large-scale interlaboratory comparison involving more than 50 laboratories worldwide. It includes extended methodological descriptions, statistical analyses, tables, and figures that complement the main manuscript. Specifically, the file reports: Detailed results of particle number and mass fraction measurements for polyethylene (PE) and polyethylene terephthalate (PET) microplastics across different size classes; Comparative performance of analytical techniques, including µ-FTIR, µ-Raman, LDIR, SEM-based counting, and thermo-analytical methods; Statistical evaluation of inter- and intra-laboratory variability following ISO 5725-2; Identification of major sources of uncertainty in microplastic analysis; Recommendations for best practices in sample preparation, instrumental settings, calibration, particle counting, and data extrapolation. Although no standalone raw datasets are provided, this Supporting Information contains comprehensive tabulated data and aggregated results that underpin the conclusions of the associated publication. The material is intended to support transparency, reproducibility, and harmonisation efforts in microplastic detection and quantification methods. Funding acknowledgement “The project 21GRD07 PlasticTrace has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States.” Metadata information:• Funder name: European Partnership on Metrology• Funder ID: 10.13039/100019599• Grant number: 21GRD07 PlasticTrace