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Given the urgency imposed by the UNEP Global Plastics Treaty and emerging safety regulations, robust methodologies are needed to assess chemical complexity and ensure the safe reuse of recycled plastics. This study introduces an advanced, multi-technique analytical framework to characterize feedstock composition and hazardous contaminants in mixed plastic waste streams (nine samples P1 to P9), including post-consumer, post-commercial, and post-industrial sources, collected from industrial recycling facilities. To mirror the uncertainty that commercial sorters face, waste samples were analysed blind—their polymer identities, fillers, and additives were undisclosed until after testing. Using a comprehensive suite of techniques—Fourier Transform Infrared Spectroscopy (FTIR), Raman spectroscopy, Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Thermogravimetry–mass spectrometry (TG–MS), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray Fluorescence (XRF), and Ion Chromatography this study identifies dominant polymers (HDPE, LDPE, PP), residual additives, non-intentionally added substances (NIAS), and inorganic contaminants such as chlorine, lead, and transition metals. The results inform tailored recycling strategies, including mechanical recycling for clean streams, pre-treatment for moderately contaminated fractions, and chemical recycling for highly contaminated or chlorine-rich plastics. This paper (Part I) focuses on the comprehensive feedstock characterization and contaminant profiling. A companion paper (Part II) presents a detailed analysis of the pyrolysates generated from these same plastic samples using comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC–TOF–MS), with an emphasis on identifying degradation products, additive-derived NIAS, and recyclability-relevant heteroatom compounds. Together, these papers offer an end-to-end understanding of contamination dynamics—from waste input to pyrolysis output—thereby informing safe and circular strategies for plastic recycling. This work provides a replicable model for identifying and mitigating toxicological risks in plastic recycling and supports regulatory compliance with emerging global standards.