Microplastics (MPs) from rubber outsoles of trail running shoes may contribute significantly to contamination in protected areas. In the natural environment, weathering processes can damage MP molecular structure and alter the mobility of inorganic and organic compounds used as additives in rubber. In this study, we characterised changes in the surface morphology, functional groups, and thermal stability of MPs weathered on and below the soil surface over 12 weeks, and analysed inorganic and organic additives in leachates (0.01M CaCl2) and bioaccessibility extracts (ethyl acetate). Weathering conditions included UVC irradiation at 25 ◦C and 80% soil moisture. Microplastics on the soil surface exhibited cracking, fragmentation, and increased extractability of zinc, sulphur, titanium and fatty acids. Microplastics below the soil surface were not significantly physically or chemically altered, however zinc leachability increased following extended weathering by up to 155%. Bioaccessibility of thiol, aromatic and cyclic organic additives decreased from both surface and sub-surface MPs over the 12 week weathering period, but there was evidence of an increase in transformation by-products. Microplastic toxicity may be significantly altered by environmental conditions and MP weathering. It is critical ecotoxicological studies use weathered MPs to assess impacts on rare and endemic species found in protected spaces.