Epoxides and phosphites are often used as additives to stabilize the properties of polymers, including bisphenol A polycarbonate (BPA-PC). We describe density functional (DF) calculations of the reactions of cyclohexene oxide (CHO, cyclohexane epoxide) and phosphites with chain segments of BPA-PC, with the aim of identifying possible reaction paths and energy barriers. The reactions of CHO with the OH-terminated PC chains and with the carbonate group are exothermic, although there is an energy barrier in each case of more than 10 kcal/mol. A comparison of results for different CHO isomers demonstrates the importance of steric effects. The reactions between the same groups of the PC chain and the phosphites 2-[2,4-bis(tert-butyl)phenoxy]-5,5-dimethyl-1,3,2-dioxaphosphorinane] (BPDD) and trimethyl phosphite (TMP), and their phosphonate isomers are characterized by large energy barriers.