Polystyrene (PS) decomposition in supercritical water (SCW) and in air was studied with the diamond anvil cell (DAC) technique coupled with microscopy and FTIR, Apparent concentrations were calculated by using digital imaging analysis. When PS + water systems (11.8-22.6 wt lo PS) were rapidly heated at a rate of 2.3 degreesC/s, the PS particle melted at 279.8-320.2 degreesC. After formation of a globule at 409.3-452.5 degreesC, the globule started to dissolve in color to yellow at 496.1 degreesC. At 570.3 degreesC and 742.5 MPa, solubility reached the maximum of 91.5 wt % (11.8 wt % PS). The soluble material was a styrenelike liquid, which was identified by IR after cooling. In isothermal runs at 400 and 450 degreesC, two heterogeneous liquid phases consisting of water and decomposed PS were found. Styrenelike liquid products were identified after the reactions. PS decomposition stages in air consisted of melting, gas generation, liquid ring configuration, and finally yellow volatile products formation at 583.2 degreesC. The results show conclusively that PS can be dissolved in SCW above 496.1 degreesC and homogenous reaction is likely to occur above 570.3 degreesC. Reactions in SCW at 400 and 450 degreesC take place in heterogeneous liquid phases, while in the PS + air system, a formed liquid ring undergoes depolymerization.