Microplastic contamination (plastics < 5 mm in diameter) has been reported worldwide in all marine matrices. Adsorption and release behaviours of hydrophobic organic contaminants (e.g., persistent organic pollutants) from microplastics have since received much needed research attention, although few studies have focused on the leachable properties of plastic polymers. Toxic plastic additives such as plasticisers are vital for the end-use application of plastic materials and are incorporated into the polymer blend during manufacturing to tailor the thermal and mechanical properties. Leaching of plasticisers from consumer products as well as from plastics retrieved from the marine environment has been shown to significantly contribute to the exposure and toxicity pathways of plasticisers to humans and wildlife. In this study, the leaching behaviours of common phthalate acid ester (e.g., DEHP) and diphenol (e.g., BPA) plasticisers from polystyrene based microplastics were analysed under controlled conditions representative of different marine ecosystems. Mathematical models were generated to determine leaching kinetics (e.g., rate-determining modes) as well as to characterise the chemical properties (e.g., polymer/plasticiser reactivity) and abiotic conditions (e.g., temperature) impacting leaching. These leaching models will allow researchers to predict plasticiser concentration in waters dosed with plastics during treatments in ecotoxicity and exposure studies, ensuring more thorough assessment of exposure pathways, toxicity responses and threshold concentrations (e.g., LD50). Furthermore, understanding the leachable properties of plastics is critical for a more accurate risk assessment of plastics to humans, the environment and wildlife.