Microplastic particles (microplastics) often contain an unknown quantity of known as well as unidentified chemicals in addition to the polymer material. Intentionally added chemicals may leach from the polymer matrix under environmental conditions, subsequently entering water bodies. There they may exert toxic effects on aquatic organisms and contribute to the overall chemical load. However, studying such effects is challenging due to the complexity of mixtures that are released, as for most commercial polymers the identity and amount of additives are undisclosed. Here we present an approach to produce well-defined additive-loaded microplastics applicable to subsequent aquatic ecotoxicity testing. We used ZnO, and commercially available Lubio products as model additives, both being relevant in industrial applications as UV-stabilizers and polymer antiaging systems, respectively.