The effects of plastic additives on depolymerization of polycarbonate (PC) in sub-critical water were examined. Depolymerization of PC with two additives was carried out in an autoclave at temperatures from 533 to 613 K for reaction times ranging from 15 to 60 min. The additives used were a flame retardant (decabromodiphenyl ether, DBDPO) and a plasticizer (di-n-octyl phthalate, DnOP). The main products of PC depolymerization in the presence or absence of the additives were bisphenol A (BPA) and phenol, which were identified by Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and gas chromatography mass spectrometry (GC–MS), and quantified by gas chromatography (GC). The addition of DBDPO accelerated the hydrolysis of PC while the addition of DnOP had the opposite effect, and both additives reduced the yield of BPA. The activation energy for PC depolymerization in sub-critical water was found to be lower with DBDPO additive than with DnOP.