The stability of polyethylene terephthalate is being studied in order to predict its long term behavior as the base of the film and tape used to record archival information. This report contains results of the first year's work. Film base, with and without photographic and other coatings, was aged at several temperatures and humidities. Mechanical and calorimetric properties and molecular weights were measured at intervals. Degradation was relatively rapid at temperatures of 115, 100, and 85° C at 100per thousand relative humidity. The scission rate at 85° C was about 10^-6 mol scission/g-day and the activation energy was 113 kJ/mol. Rates decreased strongly with relative humidity, becoming negligible in dry air and nitrogen. At 55° C and 100per thousand relative humidity degradation was not significant in 163 days. Samples were seriously embrittled by the introduction of about one scission per molecule - about 10^-4 mol scission/g. In its present state, gel permeation chromatography probably is not precise enough to be a good degradation monitor, since one would like to know when degradation had exceeded a fraction of the damaging level. Differential scanning calorimetry showed little change in melting behavior of samples aged at 55 and 85° C. Glassy samples crystallized during aging at 85° C but not at 55° C. Magnetic tapes based on poly(ethylene terephthalate) usually have a polyester polyurethane binder that holds the magnetic particles. The binder is thought to be more sensitive to hydrolysis than the tape base. Results obtained with a thermoplastic polyester polyurethane imply that there may be an equilibrium extent of degradation for any storage condition. At 20° C and 50per thousand relative humidity, about 70x 10^-5 mol scissions/g might occur. The corresponding molecular weight is only 1400 and it is expected that binder of this molecular weight would be too soft. It is anticipated that the study will go on for four more years. Agings at 35° C at several humidities are going on and samples will be examined at intervals. Attempts will be made to develop more sensitive methods to detect degradation.