The stability of polyethylene tarephthalate is being studied in order to predict its long term behavior as the base for film and magnetic tape. This report contains results of the second year's work. Film base, photographic film, and electrographic film are being aged at several temperatures and relative humidities, RH. Acid contents, mechanical properties, and molecular weight have been measured at intervals. The rate increase in acid content, k, provides a useful measure of the degradation rate. Data for films aged between 115 and 55 C at 100per thousand RH obey the equation Ln k - 39.3 - (14000/T), where k is inper thousand/day and T is the absolute temperature. The water content is included in k, which is approximately proportional to RH. Lifetimes of the films appear to be equal to 69.3/k days. Extrapolation to 25 and 20 C gives 400 and 900 years for the Lifetime at 100per thousand RH. A more sensitive analytical method than now used will be required to make useful measurements of acid content at 35 C, even after samples have been aged for five years. Two films have always failed before the others. The short-lived films might well be stored with others, since failure of the former would warn of the approaching failure of the latter. Magnetic tapes are also being aged, primarily to study the binder that holds the magnetic oxide to the substrate. Binder on five brands of tape that were aged at 50 C became crosslinked at RH of zero and 11per thousand, changed little at 30per thousand RH, but degraded at 100per thousand RH. The binder on tapes crosslink slightly. Badly degraded binder was not gummy but was easily detached from the substrate. Model binder aged 85 C and 100per thousand RH eventually became brittle.