The presence of microplastics (MPs) in sewage sludge poses environmental challenges due to their persistence and potential impact on sludge treatment processes, including hydrothermal carbonization (HTC). This study investigates the fate of common MP under conditions relevant to sewage sludge HTC processing, with a particular focus on their degradation and persistence of dissolved degradation products in the process water. Experiments were conducted over a wide temperature range and selected residence times, while also assessing the influence of reactor scale. The solid products were characterised using thermogravimetric, spectroscopic, and microscopic techniques, revealing progressive structural transformation and carbonization. Liquid filtrates were analysed photometrically for total organic carbon (TOC), chemical oxygen demand (COD), and phenol content. The results indicate that, at low to intermediate temperatures, hydrolytic depolymerisation leads to the formation of soluble organic compounds and increased TOC and COD levels in the aqueous phase. At elevated temperatures, secondary reactions promote carbon incorporation into the solid phase, resulting in increased hydrochar yield and reduced dissolved organic content in the liquid fraction. The obtained results provide important knowledge of the hydrothermal degradation behaviour of MPs and may contribute to the optimisation of thermochemical sewage sludge treatment processes. The study also highlights the role of process operational parameters and reactor scale in determining product distribution and properties.