Nonbiodegradable polymers are an environmental concern, and various techniques have been developed to recycle and reuse them. Pressurized hot water, or supercritical water, is an interesting alternative as a reaction medium for depolymerization, since water is a readily available "green" solvent and its physicochemical properties can be widely adjusted in the vicinity of the critical point. In the present study, various reaction conditions (reaction time, temperature, and use of additives and catalysts, i.e., H2O2, Pd, NaOH, and HCl) were applied to obtain as high styrene monomer yields as possible in the decomposition of industrial expandable polystyrene (EPS) in a pressurized, high-temperature aqueous medium. Other main reaction products were of interest as well. Reactions were carried out in laboratory-constructed reactors made of Hastelloy. The highest styrene recoveries, ca. 57 wt % of the initial EPS load as measured by gas chromatography-mass spectrometry (GC-MS), were obtained after NaOH addition in 20 min reaction time at 400 degrees C. Without additives, the highest yields were only ca. 13% of the initial load. Other main reaction products were acetophenone, benzaldehyde, benzene, ethylbenzene, phenol, and toluene. Thermal field-flow fractionation (ThFFF) was used after the degradation of EPS to check for possible high molar mass products. When the temperature was raised to 400 degrees C and higher, EPS residues were not detected in fractograms. Low molar mass EPS residues (M < 100.000) could not be detected because their signals were overlapped by the enlarged void peak signals due to monomer components. As a whole, the analytical techniques (GC-MS and ThFFF) employed in the characterization of EPS degradation in a pressurized, high-temperature medium complemented each other well and showed good performance.