Microplastic contamination is an ongoing problem in marine ecosystems, particularly due to the presence of toxic additives such as plasticisers. Plasticisers such as diethyhexyl phthalate (DEHP) and bisphenol A (BPA) are commonly incorporated into polymers during plastics manufacturing, however, these additives are known endocrine disruptive chemicals and can be highly toxic in the environment1. While the toxicological effects of plasticised microplastics on marine macrobiota are well understood, their relationship with the microbiome still requires further investigation. Studies exploring microbial colonisation and biofilm formation on microplastics have revealed unique bacterial communities influenced by the plastic’s morphology (e.g., surface porosity) and chemical composition (e.g., polymer/additive type)2. Certain bacterial taxa have even been found to possess pathogenic and/or plastic degrading metabolic capabilities, and can change in abundance dependent on the polymer substrates. Therefore, understanding biofilm formation on high-concern polymers (e.g., polystyrene (PS) and polyvinyl chloride (PVC)) with common additive compositions (e.g., DEHP and BPA) is vital to determine the relationship between microbiota and microplastics. To address this knowledge gap, in this study, PS and PVC microplastics – virgin and containing DEHP or BPA – were exposed in-situ to seawater representative of North-East-Queensland tropical waters. Microbial colonisation and biofilm composition was assessed using 16S high-throughput DNA sequencing and compared as a function of polymer and additive type to understand the factors impacting biofilm formation. Results indicated that biofilm community structure and specific taxa abundance is influenced by the chemical composition of the microplastic, indicating that specific taxa may be enriched on microplastics depending on their polymer-additive composition.