We investigated how two common plastics–PET (used in water bottles) and PVC (used in pipes and packaging)–release chemical additives called phthalates when they are subjected to high temperatures. Phthalates are concerning, as they can accumulate in living organisms and may harm the environment as they degrade. To study this, we placed both untreated and heat-exposed pieces of PET and PVC in clean and synthetic freshwater for 60 days. During this time, we collected water samples to measure the amount of organic material and phthalates that had been released. We used high-precision laboratory instruments to detect both the phthalates and breakdown products released from the plastic material. Our results showed that heating plastics altered the amount of chemicals released. For example, heating PET to 250ºC resulted in increased benzophenone levels, a UV-stabilizer used to protect plastic materials from sunlight-induced degradation. In contrast, one common phthalate, DBP, decreased after heating–likely due to evaporation or chemical breakdown. Understanding how the additives in PET and PVC change over time helps us better predict the potential health and environmental risks associated with plastic degradation.